feat(sdk,net): full std::net::TcpStream surface - #42

Addresses Gemini code-review feedback on astrid-runtime/sdk-rust#42.

Before:
- timeout=None falls through to a 50ms internal timeout.
- Timeout expiry with no data returned Ok(empty Vec).
- Caller can't distinguish 'transient no data' from 'peer disconnected'.

After:
- timeout=None drops the tokio::time::timeout wrapper entirely. Reads
  block until data, EOF, or capsule unload (via the outer
  bounded_block_on_cancellable cancellation token).
- Empty Vec is now ALWAYS EOF — matches std::io::Read's Ok(0)
  contract.
- 'read would block' error only surfaces when the caller has explicitly
  set a read timeout that expired. SDK-side, that maps to
  std::io::ErrorKind::WouldBlock so std::Read callers can distinguish.

The peek and write_bytes paths already had the right shape; only
net_read_bytes carried the bug.

)

## Linked Issue

Closes #745

## Summary

Adds outbound TCP to the host ABI: `net-connect-tcp` plus 14 more host
fns that give capsules full `std::net::TcpStream` parity. Unblocks every
capsule that needs persistent TCP — WebSocket clients (Fulcrum, Nostr),
MQTT, Discord/Telegram gateways, postgres / redis. Strictly additive to
the existing `net` interface; **TLS is not in scope** (capsules ship
their own crate, matching the `std::net` model where TLS lives in
user-space).

## Changes

- **WIT**: syncs canonical
[astrid-runtime/wit#5](astrid-runtime/wit#5)
into in-tree `wit/astrid-capsule.wit`. Adds the `shutdown-how` enum + 15
new `net-*` fns (`connect-tcp`, `read-bytes`, `write-bytes`, `peek`,
`shutdown`, `peer-addr`, `local-addr`, `set-nodelay` / `nodelay`,
`set-read-timeout` / `read-timeout`, `set-write-timeout` /
`write-timeout`, `set-ttl` / `ttl`) and brings in
`ipc-message.principal` (canonical PR #4) which had drifted from the
in-tree copy.
- **Manifest**: `CapabilitiesDef.net_connect: Vec<String>` — per-capsule
allowlist for outbound TCP destinations. Each entry is `"host:port"`
(exact) or `"host:*"` (any port). Empty / missing denies all outbound
TCP (fail-closed). Independent of `net_bind`, `http`, `host_process`.
- **Kernel** (`engine/wasm/host/net/`): `net_connect_tcp` runs
capability check → host string validation → DNS resolve → SSRF airlock
(`is_safe_ip`, same gate as `http-request`) → bounded 10s connect.
`read_bytes` / `write_bytes` are byte-stream variants honouring
per-stream timeouts (empty Vec = EOF unambiguously; timeout fires as
`would block` for SDK-side `ErrorKind::WouldBlock` mapping). `peek` uses
tokio's native `TcpStream::peek`. `shutdown` uses `socket2::SockRef` for
full `Read|Write|Both` direction support.
- **Security caps**: per-call read/peek buffer capped at
`MAX_BYTES_PER_CALL = 10 MB` (guards against `u32::MAX` OOM). Host
string validation (non-empty, ≤255 bytes per RFC 1035, no null bytes) at
the host-fn boundary.
- **Security gate**: new `CapsuleSecurityGate::check_net_connect` trait
method, default-deny. `ManifestSecurityGate` impl matches `host:port`
against the manifest allowlist (case-insensitive host, exact-or-`*`
port).
- **Refactors**: `net.rs` split into `net/{mod,handshake,stream}.rs` and
`manifest.rs` split into `manifest/{mod,capabilities,topics}.rs` to stay
under the 1000-line CI cap. `pub use` re-exports preserve the public
API.

## Test Plan

### Automated

- [x] `cargo test --workspace` passes (296 lib tests in astrid-capsule —
was 285, + 11 new covering net_connect pattern matching, socket2
shutdown round-trip, validate_host gates)
- [x] No new clippy warnings (`cargo clippy --workspace -- -D warnings`
clean)
- [x] `cargo +nightly fmt --all --check` clean

### Manual

- [ ] Tag a SDK release (separate PR per CLAUDE.md) so capsule authors
can bump
- [ ] Smoke-test a capsule against a public `ws://` endpoint over the
new surface (deferred to a follow-up capsule PR)

## Refs

- RFC:
[astrid-runtime/rfcs#27](astrid-runtime/rfcs#27)
- WIT:
[astrid-runtime/wit#5](astrid-runtime/wit#5)
- SDK (rust):
[astrid-runtime/sdk-rust#42](astrid-runtime/sdk-rust#42)
- SDK (js):
[astrid-runtime/sdk-js#3](astrid-runtime/sdk-js#3)

## Checklist

- [x] Linked to an issue
- [x] CHANGELOG.md updated under `[Unreleased]`

## Summary

JS/TS mirror of
[astrid-runtime/sdk-rust#42](astrid-runtime/sdk-rust#42).
Adds outbound TCP for capsules — `connect()` plus the full set of
methods a Rust binary's `std::net::TcpStream` exposes — on the
`StreamHandle` class.

## Changes

### Ambient module (`wit-imports.d.ts`)

Declares the 14 new host fns + `ShutdownHow` type in the
`astrid:capsule/[email protected]` module, matching the canonical WIT in
[astrid-runtime/wit#5](astrid-runtime/wit#5).

### `net.ts`

- `connect(host, port)` — outbound TCP (already in this branch's tip,
now upgraded from a stub).
- New `StreamHandle` methods:
- `readBytes(maxBytes)` / `writeBytes(data)` — byte-stream read/write
(no length-prefix framing).
  - `peek(maxBytes)` — non-destructive read.
  - `shutdown(how)` — half-close (`'read' | 'write' | 'both'`).
  - `peerAddr()` / `localAddr()` — `ip:port` strings.
  - `setNodelay(b)` / `nodelay()` — TCP_NODELAY.
  - `setReadTimeout(ms)` / `readTimeout()`.
  - `setWriteTimeout(ms)` / `writeTimeout()`.
  - `setTtl(n)` / `ttl()` — IP TTL.

### Submodule

`contracts/` bumped to `feat/net-connect-tcp` tip (matches WIT PR head).

### Unchanged

The existing framed surface (`recv` / `tryRecv` / `send` / `close` +
`AsyncIterable<Uint8Array>`) keeps its semantics — that's the right
shape for the inbound CLI proxy. Outbound TCP capsules use the
byte-stream pair (`readBytes` / `writeBytes`).

## TLS

Out of scope. JS-side capsules ship their own WebSocket / TLS library
over `readBytes` / `writeBytes`, same as native Rust binaries do —
std::net::TcpStream doesn't ship TLS, and we're holding that line.

## Verification

- `npm run -w packages/astrid-sdk build` clean (tsc -b)
- 64 contract types regenerate cleanly via the prebuild script

## Refs

- RFC:
[astrid-runtime/rfcs#27](astrid-runtime/rfcs#27)
- WIT:
[astrid-runtime/wit#5](astrid-runtime/wit#5)
- SDK (rust):
[astrid-runtime/sdk-rust#42](astrid-runtime/sdk-rust#42)
- Core:
[astrid-runtime/astrid#746](astrid-runtime/astrid#746)
- Tracking issue:
[astrid-runtime/astrid#745](astrid-runtime/astrid#745)