New header-only include/pineforge/drawing.hpp (all inline) backing the
codegen drawing feature: value-view handles (Line/Box/Label/Linefill),
ChartPoint value struct, per-type DrawingArena<Rec> (monotonic ids,
tombstoned slots, exact-cap FIFO eviction), and the pf_* free functions
for line/box/label/linefill geometry + the pf_noop visual sink.

Dead/na-handle access throws pine_drawing_error (: std::runtime_error),
caught by BacktestEngine::run() -> halts the backtest exactly like
TradingView. Always-on, no flag. line.get_price does real infinite-line
interpolation (throws on xloc.bar_time, na on degenerate). copy() is a
deep arena copy; delete() is a silent no-op on na/dead.

tests/test_drawing.cpp (ctest) covers alloc/monotonic ids, mutate-through
aliased handle, deep-copy independence, dead-handle throw, FIFO eviction,
get_price interp/degenerate/bar_time, chart.point value copy, na<Handle>().
Nothing here is exported via the C ABI (codegen-emitted in-process calls).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>