fix: level-gate the call-graph solve at -a 1

[rahlk]

Type: fix · Branch: fix/issue-31-level-gate-callgraph · Part of epic #26

Problem

provider.build(...) runs unconditionally (src/core.ts:34), including at -a 1 where the emitter discards the result (emission gated to level >= 2, emit.ts:345). The heavier Jelly leg runs for nothing; this violates "the cheap path stays cheap" (dataflow-graphs.md:208).

Change

const cg = opts.analysisLevel >= 2
 ? provider.build({ project, symbol_table, root: opts.input, log, phantoms: opts.phantoms })
 : { edges: [], external_symbols: {}, synthesized_callables: {} };

L3+ is unaffected (L3 ≥ 2). Verify no L1 emit path reads provider externals/synthesized (schema declares both L2 → expected safe).

Acceptance

• The call-graph solve (incl. Jelly) does not run at -a 1.
• -a 2/3/4 output unchanged; the monotonicity gate proves -a 1 output is byte-invariant.

Files: src/core.ts, lock-in test.