Add expandForValueSet to CodeSystemProvider for SQL-backed expansion

[jmandel]

Summary

Adds one optional method to the CodeSystemProvider interface — expandForValueSet(spec) — that lets SQL-backed providers handle ValueSet expansion in a single query instead of the per-code iterator loop.

The worker groups compose includes/excludes by code system and passes the full hull (includes, excludes, activeOnly, paging) to the provider. Providers that implement this method return an async iterable of fully-resolved entries; the worker feeds them through includeCode() for dedup, import filtering, and FHIR construction. Providers that don't implement it (e.g., SNOMED) are completely unaffected — the existing iterator-oracle path runs unchanged.

What changed

Interface (tx/cs/cs-api.js)

• New optional expandForValueSet(spec) method with JSDoc contract
• Returns AsyncIterable<ExpandedEntry> or null (fall back to existing path)

Worker (tx/workers/expand.js)

• Groups includes/excludes by code system before processing
• Calls expandForValueSet first; marks handled systems to skip framework's manual include/exclude loops
• Paging offset/count only passed for single-system composes (multi-system gets null — framework applies paging in finalization)

RxNorm (tx/cs/cs-rxnorm.js)

• expandForValueSet using better-sqlite3 lazy cursors
• TTY filter → WHERE IN, STY filter → JOIN rxnsty, concepts → WHERE RXCUI IN
• GROUP BY RXCUI when JOINs cause row multiplication
• Excludes, activeOnly, searchText pushed into SQL

LOINC (tx/cs/cs-loinc.js)

• expandForValueSet using better-sqlite3 lazy cursors
• Relationship filters → JOIN on Relationships table
• STATUS → direct WHERE, CLASSTYPE → JOIN through Properties+PropertyValues
• LIST/answers-for → reversed relationship JOIN
• Multiple includes → UNION ALL with GROUP BY dedup

Other

• Perf counters (tx/perf-counters.js) for timing expandForValueSet paths
• locateMany / filterPage stubs on SNOMED, LOINC (may be useful for future IPC)
• Bug fixes: searchFilter arg order, validate.js messages, TxParameters.assign paging fields

Benchmark results

RxNorm (13 tests, all pass, median 37x speedup):

Test	Opt	Base	Speedup
filter-tty-sbd	6.8ms	249ms	37x
filter-tty-in-multi	1.4ms	476ms	342x
paged-offset-100	1.3ms	228ms	177x
text-aspirin	1.8ms	TIMEOUT	∞
filter-sty-t200	217ms	1522ms	7x

LOINC (14 tests, all pass, median 25x speedup):

Test	Opt	Base	Speedup
filter-status-active	50ms	5325ms	106x
filter-class-chem	14ms	941ms	68x
filter-scale-qn	51ms	2611ms	51x
filter-classtype-lab	82ms	2021ms	25x

LOINC jest: 37/37 pass. Replay: no regressions on captured production queries.

Design doc

See docs/PROVIDER_INTERFACE_EVOLUTION.md for full rationale, SQL strategies, paging contract, and index decisions.