ABI: deliver pinned process signal events

Parent: #140 Profiles: #163, #164 Roadmap: #134

Implement the shared pending Bun__onSignalForJS boundary against zig-js's real process EventEmitter. Pin the Bun 4982b91 signal map and event behavior; do not expose a link-only stub.

Acceptance

• Pin the exact BunProcess implementation source and cross-platform signal-name mapping.
• Map supported native signal numbers to the canonical SIG* event names in Bun insertion order.
• Emit synchronously on only the selected realm's process, with exact (signalName, signalNumber) arguments.
• Preserve listener ordering, once/removal behavior, and listener-thrown exceptions through the private pending-exception boundary.
• Reject null/dead globals, unknown signal numbers, and pre-existing pending exceptions without observable mutation.
• Add focused unit and compiled Home/Bun profile coverage in Debug and ReleaseSafe.
• Refresh exact inventories and concise README/API/ABI counts.
• Record the bounded fixing CI and close.

Backward compatibility is intentionally out of scope.

Implemented in 99433614 and documented in a856d0ec. Focused semantics and both external profile fixtures pass in Debug and ReleaseSafe; Bun/Home audits, contract validation, and docs build pass. Counts are now Home 424/471 (47 pending), Bun 416/461 (45 pending). Awaiting fixing main CI.

Completed on main through 99433614 and a856d0ec. Canonical platform signal mapping, realm-local synchronous EventEmitter delivery, once/removal ordering, thrown-listener propagation, and dual-profile evidence are green in the fixing private-abi-boundary job.