ABI: implement exact no-side-effects value stringification

[chrisbbreuer]

Parent: #134
Related: #140, #163, #164

Goal

Implement the shared pinned Bun__noSideEffectsToString boundary used by fatal and unhandled-error reporting.

Acceptance

• Return exact strings for undefined, null, booleans, int32/double values, BigInts, and described/undescribed Symbols.
• Return string inputs unchanged and collapse every other object/function/error/proxy to [object Object].
• Never invoke conversion hooks, getters, proxy traps, or mutable globals.
• Validate VM/global ownership and preserve existing pending-exception state.
• Add focused primitive/large-BigInt/Symbol/proxy/identity/foreign coverage and a compile-link-runtime fixture call.
• Refresh Home/Bun inventories with zero unclassified symbols.
• Update README and compatibility docs, publish small implementation/docs commits on origin/main, and close only after evidence is live.

[chrisbbreuer]

Implemented and published in two direct main commits:

• 8a5391ec — exact pure primitive/BigInt/Symbol formatting, raw string identity, VM-small-string stability, object collapse, wrong-VM rejection, zero proxy/conversion observability, pending-state preservation, focused coverage, fixture linkage, and refreshed inventories
• 53d471db — README and compatibility evidence/count refresh

Verification:

• focused pure-stringification matrix: 3/3 passed, 0 leaked
• Home fixture: 227/227 symbols linked and runtime matrix passed
• Home inventory: 228/431 implemented, 203 pending
• Bun core inventory: 222/421 implemented, 199 pending
• public/private audit: 117 public, 19 extensions, 232 private/support exports
• zero unclassified inventory entries

The benchmark artifact remains 36a617b3: this diagnostic ABI path does not change measured execution performance.