ABI: implement exact JSFunction source and tier-up controls

[chrisbbreuer]

Parent: #134
Profile umbrella: #140
Home inventory: #163
Bun inventory: #164
Builds on: #221

Implement the pinned shared private-JSC exports:

• JSC__JSFunction__getSourceCode
• JSC__JSFunction__optimizeSoon

Acceptance criteria:

• Return the exact parser-captured source span for ordinary functions, arrows, methods, generators, async functions, and class constructors without invoking user code.
• Return false for native, bound, non-function, invalid, and exception cells while leaving the output unchanged.
• Publish a stable group-lifetime ZigString view with exact Latin-1/UTF-16 tagging and UTF-16 length.
• Make optimizeSoon schedule the next invocation of an eligible bytecode function for native-tier consideration without compiling synchronously.
• Leave already-ready/rejected tiers, tree-walk functions, native/bound functions, invalid values, and pending VM exceptions unchanged.
• Focused tests cover source fidelity, Unicode source, rejection cases, next-invocation tier transition, VM ownership, and exception neutrality.
• Extend the Home consumer fixture and publish exact Home/Bun inventory, ABI documentation, README, and parent counts.
• Batch the focused test, consumer fixture, both private audits, and C-API classification before publishing.
• Leave benchmark scores unchanged unless a benchmark path is rerun.

Completed evidence

Implemented on main in a30b91b8; pinned inventories, ABI documentation, and README counts were published in 12ff7451. Source retrieval now returns parser-exact group-lifetime ZigString views for ordinary, arrow, method, generator, async, Unicode, and class functions while rejecting native, bound, non-function, invalid, and exception cells without output mutation. Tier-up requests remain scheduling-only and prime the next eligible cold bytecode invocation without synchronous allocation or code generation; settled tiers and ineligible functions remain unchanged.

The intentionally batched verification passed 15/15: focused tests passed 3/3 with zero leaks, the Home fixture linked and exercised 203/203 symbols, Home is 204/431 (227 pending), Bun core is 198/421 (223 pending), and the public audit reports 117 public, 19 extensions, and 208 private/support exports. Benchmark scores remain unchanged because no benchmark path was affected or rerun.

[chrisbbreuer]

Completed on main in a30b91b8, with inventories and documentation in 12ff7451. All acceptance criteria are checked; the recorded batch passed 15/15 with zero leaks, fixture 203/203, Home 204/431, Bun 198/421, and 208 private/support exports.