docs: mutation-class catalog, padding-convention note, integer-only requant open problem, SYM pool mechanics

## Problem

Four pieces of decided-but-scattered knowledge have no home in `docs/CONVENTIONS.md`:

1. **Mutation-class catalog** — scale-blindness, eps-vacuity, layout-divergence, uniform-dy-vacuity, freeze-the-scale. Currently spread across generator docstrings, C test comments, and commit messages; every new SYM gold suite re-derives them from scratch.
2. **SAME-vs-EXPLICIT padding note** — C `SAME` is TF-minimal padding and diverges from PyTorch's symmetric padding for stride > 1 / even kernels; PyTorch-parity weight loading needs `EXPLICIT`. Bit us in the v2 examples; never written down.
3. **Integer-only requant open-problem entry** — Deutel et al. (arXiv:2407.10734) leave the requant realization open and use the FPU extensively; ODT's requant primitives are float-path. Per the research-backed-numerics rule an int-multiplier+shift scheme must NOT be homegrown — document it as an open problem with the literature pointer.
4. **SYM pool mechanics note** — Max = select, Avg = scale-fold, Adaptive = rounded integer division (#205), in the style of the existing "two accumulation schemes" note.

## Proposed work

One docs PR against `docs/CONVENTIONS.md`, parallelizable with everything; item 4 may land with the pools PR instead if M4 ships first.

## Relations

#192, #137, #205.

🤖 Generated with [Claude Code](https://claude.com/claude-code)