CL-8980: resume truncated reads by same path plus offset

[TheGreatAxios]

Implements the operator-greenlit single canonical resume for CL-8980: same path plus explicit offset, nothing else.

This supersedes #1172 (self-describing cursor handles): branched fresh from main, and the one-shot continuation-handle machinery (mint/decode/replay map, spent-handle errors) is removed instead of hardened.

• Truncated reads keep the plain "Use offset=N to continue." notice; every read is a stateless idempotent ranged read, so verbatim follows work on first use, replay, and fresh instances after compaction/session resume.
• Dead offsets fail as range errors naming the file (or spill URI) and the valid range.
• Chunked same-path reads carry rising offsets and key distinctly in thrash bookkeeping: one ranged read per window, not a same-path loop.
• Compaction keeps the newest window whole with its notice; distinct windows never hollow each other.
• Spill notices (result truncation, background-shell spill) name read_file with the URI plus offset/limit.

Red-then-green: red commit 6d1925f (5 failing), green commit fe0f23c.

Verification: bun run typecheck exit 0; bun test ./src ./tests ./evals ./scripts --randomize --seed 424242: 8035 pass 0 fail; bun run lint exit 0.

Fixes CL-8980

[linear-code]

CL-8980