Build reflection/Dynkin research corpus and compiler sketch

[isomorphisms]

What this adds

• Federico Ardila's official Coxeter-groups YouTube/course/index links, including the Lecture 8 correction warning.
• Sourcing/licensing notes and original chapter-level orientation for Humphreys, Reflection Groups and Coxeter Groups; the copyrighted book itself is not mirrored.
• A broader reflection/Coxeter reading list.
• Summaries of John Baez's main ADE/Dynkin sequences and compiler-oriented takeaways.
• Householder-reflector notes connecting Coxeter/reflection structure to numerical linear algebra.
• An open-source survey covering Sage, coxeter3, CHEVIE, GAP, LAPACK, Eigen, egg/egglog, TensorRight, and related implementations.
• An Idriç-shaped pseudocode sketch for Coxeter systems, Dynkin/Cartan data, generator words, Householder reflectors, and proof-carrying rewrites.
• A compiler rewrite sketch centered on preserving certified group structure before lowering to dense matrix multiplication.
• A first executable exact reducer for A2/A3 with deterministic reduced words and replayable involution/commute/braid evidence.
• Exhaustive small-word tests, tamper/replay tests, long-input coverage, and a small Sage/coxeter3 behavioral-oracle script.

Integration tracking

• ai-ci issue #15 now tracks observing Coxeter/reflection behavior: Start observing Coxeter reflection/group-theory behavior isomorphisms/ai-ci#15
• The intended RHS-verification integration issue could not be created because GitHub Issues are disabled in isomorphisms/the-equality-sign-means-equality; its proposed title/body are preserved under issues/rhs-verification-import-coxeter.md.

First executable slice

Implemented in reducer.py and deliberately limited to A2/A3.

The reducer uses the exact type-A permutation/inversion model only to decide whether appending a generator raises or lowers Coxeter length. Every actual word transformation is an explicit Coxeter rewrite recorded in a replayable evidence chain. A3's longest reduced word has length 6, so the only search is a bounded braid-class BFS of length at most 7 to expose a cancellation or select the lexicographically least reduced expression.

tests/test_reducer.py checks exact element preservation, minimal length, full evidence replay, evidence tamper rejection, exhaustive short inputs, canonical output, and 1000-generator inputs.

oracles/sage_compare.py compares the returned element and length against Sage's reflection backend and repeats the fixtures with Sage's optional implementation="coxeter3" when available. It intentionally does not require Sage/coxeter3 to choose the same reduced spelling.

This PR still does not introduce a general Coxeter-matrix reducer, minimal-root automaton, or equality saturation.