Complete mixed BIT fallback descriptors and stream adapters

[jmbarrancoml]

I complete the mixed fallback shear descriptors left open by commdata2338's PR2188 and connect them to the sequential ADD from PR2171. Each shear retains its intermediate original-row permutation through inverse elimination; physical execution uses the orientation of the actual target after upper-to-lower conversion.

Two selected examples distinguish the rules. An upper shear needs physical coefficients [16,21,21], while the original target orientation gives [21,21,16]. A path with good and fallback ancestors needs coefficient 9 mod25; low-only ancestry gives 24. I execute both comparisons on opaque payload streams.

Evidence:

• Eight complete 128-coordinate operators match direct weighted matrices, retaining all 16,256 shear slots per fixture.
• Two complete small payload programs execute 1,338,444 and 85,293 bits, with zero/nonunit ADDs, fresh stock and charged reversal children.
• Generated mixed descriptors supply the remaining child weights. I record each completed-child oracle's width and full logical volume, retaining the 98,682-child allowance.
• I apply the existing unequal-buffer parking contract to the selected live/private inventory and state its locality, descriptor-copy and return requirements.
• Registered Python 3.12 replay passed normally and with -O; source pins and deterministic receipt comparison passed.

python3.12 scripts/research_package.py --package research/selected-bit-fallback-streams

Scope: conditional composition proof and executable adapters. The remaining weighted children use exact, charged oracles in the payload tests. Their physical recursive implementations, further local-frame bindings, full caller/descriptor-stack integration and finite routing admission remain open. I claim no new exponent or complete multiplication theorem.

The final overlap check covered 445 open PRs and 896 Discussions with complete pagination. I recorded 1337isnot1337's new same-family three-route invoice claim as separate work; this package retains that cost interface.

I credit Ross Richardson's PR2159 covariance and fallback schedule, commdata2338's PR2160/2188 normal-form and mixed descriptor work, the source527 framework, and PR1379/1641 parking results. Three unchanged predecessor files retain their notices and exact hashes under vendor/. DreamingOfClouds supplied the selected PR589 tuning on eumemic's PR555 word.

Coordination: Discussion 2198. Base: canonical main 0fdd7d82de03e43c9e5a1000dfbcd380d1b687d2, priority revision 4. I contributed as jmbarrancoml with substantial OpenAI Codex assistance; no independent human or proof-assistant verification claimed.