Add file-backed P/2 fact sources to the JavaScript WAM

[s243a]

Closes G-W4: the JS WAM had only an empty fact_sources: {} scaffold, so every fact had to be compiled inline.

What changed

Lightweight file-backed P/2 facts, modeled on Lua's lua_fact_sources/1:

• Option javascript_wam_fact_sources([source(P/2, file(Path))]) (alias js_fact_sources/1).
• Matching predicates emit I.CallFactStream + I.Proceed() instead of inline WAM.
• Node runtime reads TSV/CSV (tab if present, else comma) or JSONL ([a,b], {a1,a2}, {args:[...]}) via fs.readFileSync. No npm deps.
• First-arg index when A1 is bound; multi-solution via the existing fact_stream choice point.
• Cell values go through parse_term (int/float/atom/compound).
• Inline-facts path (no option) is unchanged.

LMDB / CSR are out of scope (same as Lua).

Tests (green locally)

swipl -q -g run_tests -t halt tests/test_wam_javascript_fact_sources.pl
swipl -q -g run_tests -t halt tests/test_wam_javascript_builtins.pl
swipl -q -g run_tests -t halt tests/test_wam_javascript_lowered.pl

Parser probes, 48-query local suite, and lowered emitter stay green.

Branching

Branched from grok/wamjs-parser and extended it (net additions). Shared files are not edited (INTEGRATION_PATCH.md unchanged).