JAB-006: buf.cpp violates the documented API.md buffer contract — silent truncation + missing unmap primitives

Now: OPEN

Two jab-runtime (src/jab) binding gaps surfaced by the 2026-07-07 be/ review (BE/REVIEW), where be/ JS relies on a contract buf.cpp does not honor:

buf.cpp:48 — Buf.feedStr SILENTLY TRUNCATES when IDLE is full, violating API.md's NOROOM contract (callers assume all-or-nothing + a NOROOM signal). Silent short writes corrupt output with no error.
buf.cpp:124 — API.md documents io.book / b.close() / b.trim() that buf.cpp does NOT implement — there is no deterministic unmap primitive, which is exactly what the mmap-leak class BE-017 needs to release mappings in the resident loop.

Method Issues.

Input

Context

be/ code trusts API.md: feedStr callers branch on a NOROOM return that never comes; the leak-fix in BE-017 wants io.book/close to release a mapping deterministically, but the primitives are absent.
These are jab-SIDE (binding) defects, split from the be/ review so the fix lands in the runtime, not the extension.

Goals

Buf.feedStr returns NOROOM (or throws) on insufficient IDLE, never a silent partial write — matching API.md.
Either implement io.book/b.close()/b.trim() per API.md (deterministic unmap) OR correct API.md to what the runtime actually provides; BE-017 depends on the ruling.

Constraints

Repro-first: a feedStr into a too-small buffer asserts NOROOM; an unmap primitive (if implemented) releases a mapping observably.
ABC buffer/NOROOM conventions.

WIP

Design decisions

(pending — implement the primitives vs amend API.md)

TODOs

Repro: feedStr overflow → NOROOM.
Fix feedStr truncation; resolve io.book/close/trim (implement or de-document); unblock BE-017.

Blockers and bummers

BE-017 (be/ mmap leaks) is gated on the unmap-primitive ruling here.
2026-07-24: was mislabeled DONE; jab log shows no landing and buf.cpp:48 still truncates — reopened. Sibling mapped-Buf grow gap split to JAB-017.

Outcome

(pending)