feat: build host-import modules in Rust from shape data (#37)

[schplitt]

Closes #37.

What

Host modules no longer reach the runtime as TS-generated ESM source. The client ships the module shape as plain data (source modules unchanged; host modules as a tree of function-leaf markers, data leaves as WireValues, nested objects), and the Rust runtime builds the module natively.

How the module is built (import.meta indirection)

The runtime emits only a fixed-shape template — export const search = import.meta.__iso4[0]; with identifier-validated export names and integer indices. Every value is built natively:

• data leaves via the existing wire_to_v8_value
• function leaves as async trampolines from a fixed factory, with the handle ID passed as a number, never printed into source

The values array is attached through V8's import-meta callback. Host modules therefore stay ordinary source-text modules, which keeps prefix snapshots working with no external-reference bookkeeping (the documented import { search } from 'tools:x'; globalThis.search = search prefix pattern is covered by new Rust tests, including a full snapshot roundtrip). Synthetic V8 modules were deliberately not used — their native evaluation-steps pointer would need external references on snapshot create and restore, where a miss is a crash.

Rust owns the handle table

Handle IDs are assigned in tree-walk order over the declared bindings and never leave the process. The __iso4_call dispatcher stub is now installed by the runtime itself (the client no longer sends a def for it); it resolves the leading ID before the frame is written, so:

• BridgeCall frames carry targetKind: import + resolved specifier and leaf path (the previously-dead protocol fields)
• bridge-call records carry fully resolved names (tools:search.query; shim stubs resolve to their public name Rust-side too)
• bridge-report.ts (client-side name resolution) is deleted

Rebind validation unified in Rust

PrefixRun sends only rebind locations (specifier + leaf path); the runtime validates them against the shape stored with the prefix and rejects undeclared paths, data leaves, and source modules with ERR_UNDECLARED_BINDING — the same enforcement point as undeclared globals, no longer skippable by a non-TS client. The client keeps compile-time RebindImports<M> plus the client-only checks (non-function rebind values, string overrides).

Notes

• Wire protocol change (ImportBinding is now tagged source/host; PrefixRun imports become ImportRebind entries; BridgeCallRecord drops importHandleId/rawName for a resolved name). Protocol version stays at 1 per pre-release policy.
• docs/protocol.md §5.2/§5.4/§5.6 and DESIGN.md (§4.3, §11.4, phase table — Phase 12 superseded) updated; also fixed a stale DESIGN.md claim that Date is a supported data leaf.
• Behavior nuance: rebinding an undeclared specifier with a non-function value now reports "can only be rebound with a function" (client-side) instead of "was not declared" — declared-shape knowledge lives in Rust now.

Tests

• Rust: 227 pass (10 new: data/function/nested leaves, resolved-name frames, cross-module handle offsets, invalid-handle catchable rejection, import.meta isolation from user code, snapshot roundtrips, walk-order unit test)
• TS: 465 + 83 pass; imports.test.ts rewritten for the data-tree API; new integration tests for Rust-side rebind enforcement incl. the data-leaf-rebound-with-a-function case and a positive rebind roundtrip
• clippy delta vs main: −3 pre-existing errors, +0