feat(snn-head): add opt-in three-chip resident lifecycle - #9

Add opt-in three-chip residency through SNN_HEAD_RESIDENT=1. Initialization prepares and loads all five layers once; repeated complete eight-timestep chunks attach each layer, INIT its state and run without reloading weights. CPU preprocessing and layer ordering stay in the SNN application. Default builds retain the single-chip path.

The module adds snn_head_initialize(), resident lifecycle state, selected assets/build flags, a dedicated linker and application tests. It uses one shared runtime receiver and is not reentrant. Errors stop later layers and subsequent requests. This batch contains application logic only; SRAM auditing and portable host-record parsing follow separately.

Validation: on the actual split runtime chain, the clean snapshot passes resident ASan/UBSan tests (2/2), existing SNN host tests (4/4), focused runtime tests and output-layout regressions. Single-chip stats-off/on and resident N307FD cross-builds pass; stack symbols are 4096 bytes, all five selected PBCA match their embedded ELF bytes, and 15 CPU parameter identities are preserved. Changed C/C++ formatting and whitespace checks pass.

Draft dependency: exact reception still has a potentially blocking FIFO MMIO read inside IRQ. The README states the required architecture review and historical SOC lane0/communication preparation. No new board performance or repeatability acceptance follows from these host/build results.

This is batch 8/9 replacing #1. Review only this delta against codex/pr-snn-three-assets; merge predecessors first, then retarget/rebase onto refreshed main while keeping the scope separate. Do not merge into the predecessor branch or delete it before retargeting its dependent PR. Colleagues own review and merging; no automatic merge is enabled.

Source head: 8cd3fa32d33413176906785c315836321b23b1f0; source tree: 6894d2adb5a82362c0bcdd8a6569783d0be5fa12.

Dependency: #8. Ordered replacement series: #2 → #3 → #4 → #5 → #6 → #7 → #8 → #9 → #10.

CI status at submission: the repository pre-commit.ci check reports ERROR, as it also did on original #1. The reported message is error during ci config; repository CI configuration needs separate attention. The focused remote host and cross-build results above do not mean all GitHub checks passed.