Extends the PhoundPlugin's SQLite database with two new tables: signatures and parameters
These store function/method signatures, property types, constant types, and parameter details for all user-defined code elements.

This enables external tools to perform instant signature lookups from the Phound database without requiring the Phan daemon to be running.
See #5459 for one such external tool.

Changes

• signatures table: Stores FQSEN, kind (function/method/property/constant), class FQSEN, name, return/property type, static flag, visibility, file path, line number, and docblock
• parameters table: Stores FQSEN, parameter index, name, type, variadic/reference/optional flags, and default value representation
• Indexes: signatures_name (for name-based search) and signatures_class (for class-scoped queries)
• finalizeProcess() now accepts CodeBase parameter and calls writeSignatures() after relationship flattening
• Signature data is written using individual prepared statements at finalization time (not bulk inserts, runs once, not performance-critical)

What gets stored

• All user-defined functions (from getFunctionMap())
• All user-defined class methods, properties, and constants
• Skips isPHPInternal() elements
• Skips inherited elements (where getDefiningFQSEN() class differs from current class)

Example queries

-- Find a method signature
SELECT * FROM signatures WHERE fqsen = '\Phan\CodeBase::getMethodByFQSEN';

-- Get parameters for a function
SELECT * FROM parameters WHERE fqsen = '\Phan\CodeBase::getMethodByFQSEN' ORDER BY idx;

-- Search for symbols by name
SELECT fqsen, kind, filepath, lineno FROM signatures WHERE name LIKE '%UnionType%';

-- List all symbols in a file
SELECT fqsen, kind, visibility, lineno FROM signatures WHERE filepath = 'src/Phan/CodeBase.php' ORDER BY lineno;

-- Find potentially unused methods (no callsites)
SELECT s.fqsen, s.filepath, s.lineno
FROM signatures s
LEFT JOIN callsites c ON s.fqsen = c.element AND c.type = 'method'
WHERE c.element IS NULL AND s.kind = 'method'
ORDER BY s.filepath, s.lineno;