conformance: solc and soldb emit numeric resource ids, but the spec now requires strings

[gnidan]

As of #288, the spec wil require resource identifiers to be strings: schema:ethdebug/format/materials/id is type: string (previously number | string), and type/reference.id is string-only. These constraints land with the ids-to-strings spec change.

Two external implementations exercised by the conformance suite still emit numeric ids, so their live output no longer validates against the schema.

• soldb (walnuthq/soldb, which the conformance workflow checks out at main) assumes numeric source ids throughout, not at a single serialization site: crates/soldb-ethdebug/src/metadata.rs declares struct SourceLocation { source_id: u64, … }, parses ethdebug with code.get("source")?.get("id")?.as_u64()? — so a string id makes as_u64() return None and source resolution fails silently rather than erroring — and keys its sources as BTreeMap<u64, String>.
• solc (checked out at develop) emits numeric source indices, which soldb consumes.

Both are pinned to moving branches this repo doesn't control (SOLIDITY_REF=develop, SOLDB_REF=main), so the mismatch can't be resolved here alone; it needs coordinated upstream changes to solc's ETHDebug emitter and to soldb's u64 plumbing.

Until then, a transitional carve-out keeps the conformance suite runnable: packages/conformance/src/runner.ts (validateSchema) downgrades exactly the numeric-materials/id validation failure on external output to a loud warning that points back to this issue. Every other validation failure still fails hard, and no data is rewritten before validation. The in-repo consumer (bugc) already emits string ids.

Once solc and soldb emit string resource ids, close out the transition:

• Remove the conformance carve-out in packages/conformance/src/runner.ts (validateSchema), so the suite validates ids strictly again.
• Tighten the format package's Materials.Id TypeScript type from string | number to string — it was left lenient during the transition so the type matches the carve-out's tolerance.