fix: byte-sliceable L3 statement nodes #28

Type: fix · Branch: fix/issue-28-l3-byte-spans · Part of epic #26

Problem

GraphNode (src/schema/graphs.ts:30-37) carries only line/column — no offsets — so spanOf hardcodes bytes: [0, 0] (src/schema/v2/dataflow.ts:74-75). Every L3 statement node emits [0,0], so module.source.slice(bytes) returns "". The "every node's text is a slice" contract (canonical-schema.md:148-159) holds for L1 call nodes but breaks for all L3 body nodes.

Change

Add start_offset / end_offset (UTF-16 char offsets, matching the L1 convention per model.ts:16) to GraphNode; populate them at CFG construction (src/dataflow/cfg.ts, from ts-morph .getStart() / .getEnd()). Entry/exit = whole-callable span; param = param-decl span. spanOf returns bytes: [start_offset, end_offset].

Acceptance

• For a sampled L3 statement, module.source.slice(...span.bytes) is non-empty and equals the statement text (asserted in the L3 gate).
• The monotonicity gate (sibling issue) stays green.

Files: src/schema/graphs.ts, src/dataflow/cfg.ts, src/schema/v2/dataflow.ts, test/schema-v2.test.ts.