Implement shadow rendering and fingerprinting

[sjquant]

Why

• add deterministic tracked root-instruction shadow rendering primitives
• record reusable overlay and rendered fingerprints for later shadow lifecycle work
• lock down marker, newline, idempotence, and manual-edit detection behavior with tests

Changes

• add tracked shadow render helpers and fingerprint generation in src/root-instruction-render.ts
• guard replace rendering behind explicit confirmation while supporting append and prepend
• add focused root-instruction renderer tests for marker format, newline normalization, fingerprint stability, and rendered-fingerprint edit detection