feat(lab): index-treatment arm - scoped-reread clause + gated index attachment

Two fixes to the arm under development, per Eli 2026-08-06. (1) The
upstream read-once clause inherited by every mike arm fights the scoped
design - trace mining caught a model widening a window specifically to
honor it - so the treatment prompt replaces it with a scoped norm (never
re-request the same bytes; several different windows per document are
normal; size windows to the section). Byte-exact replacement with a
loud module-load assert if upstream rewording ever orphans it; frozen
index arms keep the original clause untouched. (2) Completes the 'no
empty indexes' design: MIKE_INDEX_ATTACH_GATED=1 serves a document as
bare markdown when its derived index carries entries but no usable @N
addresses (20.6% of index bytes corpus-wide were such dead weight);
index=true on gated docs answers with the existing typed no-index note;
read gating unchanged. Surface receipt + conformance carry
index_attach_gated (true for the treatment arm, asserted false across
the frozen index family). Probe 13/13, tsc clean, suites 39/39.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pfUVhNFTRvhYGXBwoKNj6