Tracker: Phase 1 — land the license/cleanup stack

[krystophny]

Phase 1 lands the license/cleanup stack and proves the shipped libneo
build is free of GSL and FFTW links and of GPL/LGPL source. All four
leaves are unblocked today except where a Blocked-by names another.

Umbrella: #291.

Leaves

• Fix Build and test failure on PR #290 (math-kit) #292 Fix Build and test failure on PR Add clean-room math kit; drop FFTW link and LGPL quadrature #290 (math-kit). Model: sonnet.
• Fix gate job failure on PR #278 (downstream reverse-dependency gate) #293 Fix gate job failure on PR Per-PR reverse-dependency gate against downstream codes #278 (downstream reverse-dependency
  gate). Model: opus.
• Merge the license stack: #286, then retarget and merge #289 and #290 #294 Merge the license stack: Drop GSL dependency from collision_freqs #286, then retarget and merge Document third-party licenses #289 and
  Add clean-room math kit; drop FFTW link and LGPL quadrature #290. Model: haiku. Blocked by Fix Build and test failure on PR #290 (math-kit) #292.
• Verify no shipped libneo target links GSL or FFTW and no GPL/LGPL source remains #295 Verify no shipped libneo target links GSL or FFTW and no
  GPL/LGPL source remains. Model: sonnet. Blocked by Merge the license stack: #286, then retarget and merge #289 and #290 #294.

PR #278's gate fix is #293. It clears the downstream gate so later
extraction work can be verified against consumers.

Closes on completion

#285, #287, #288 close when the PR stack (#286, #289, #290) merges.

Execution conventions

Model ladder (implementation):

• haiku: label/milestone housekeeping, checklist updates, re-export boilerplate. No algorithms.
• sonnet: fully specified ports (source path + SHA given), wrappers, CI fixes with a clear failing log.
• opus: build-system and CI scaffolding, multi-file refactors, ports that change APIs.
• fable: API design issues, clean-room algorithm work, review of opus-implemented PRs.

Review ladder: sonnet PRs reviewed by opus; opus PRs reviewed by fable; haiku changes gated by CI only.

PR rules:

• One leaf issue = one PR. Branch from the base named in the issue.
• PR title = issue title. Body: Closes #N plus a ## Verification section with real failing-before and passing-after output.
• Squash merge. Stacked PRs only where the issue names a non-main base.

Executor rule: if any issue under "Blocked by" is open, stop and report instead of starting.