Adding a function note

A note records one thing about an Excel function that someone matching Excel, or explaining an Excel result, needs to know. Notes are harvested in batches from findings that have already landed (reports, evidence, product commits). Writing a note is never a step of a fix, and no other repository's work or commit depends on one.

1. Copy an existing content/notes/FN-*.json and rename it FN-<FUNCTION>-<nnn>.json (the next free number for that function; <FUNCTION> as in data/functions/FUNC.<FUNCTION>.json).
2. Fill it in:
   • functions: every FUNC.* id it applies to, sorted.
   • kind: quirk, excel-differs-from-math (needs a repro with correct, the exact mathematically correct value), bit-matching-detail, or documentation-differs (needs documentation).
   • statement: what Excel does, with its scope. Only what the evidence shows.
   • status: measured (read off recorded Excel answers) or inferred (goes beyond them; put the inferred part in inferred). basis says what the status rests on.
   • repros: the call, the exact inputs (display must parse to bits), Excel's answer, recorded_in (Excel build, platform, CPU) and source (where the reading is kept).
   • evidence: repository, full commit, path, locator, for the product commit, research write-up and readings. Evidence that exists only in a working folder is copied to content/notes/evidence/<date>/ first and cited at that commit.
3. Run dotnet run --project tools/efh/Efh.Rubric.Cli -- notes render from the repository root. It checks the notes and writes the Notes section of each named function page, NOTES.md and site/api/notes.json. notes check does the same without writing.
4. Run dotnet test tests/Efh.Rubric.Tests and commit.

Do not use the equality phrases CHARTER 7.8 forbids; say what matched, on how many rows, in which build. The format is tools/schemas/f20-function-note.schema.json.