spb/metrika Public

- parser: (endog = instruments) varlist groups -> ZQIVSpec; digit-led
  sub-commands (2sls) reassembled from number+identifier tokens via
  column adjacency
- ZQFixedEffects: within estimator with Stata conventions (add-back
  means, reported _cons, df = N-K-G), within R-squared, panel-clustered
  VCE with G/(G-1) and t on G-1 df; v0.2 restriction: cluster variable
  must equal the panel variable
- ZQIV: 2SLS via thin-Q projection of the instrument matrix (Z'Z never
  formed), residuals from original regressors, Stata 'small' inference,
  classical/HC1/cluster VCE built on projected regressors
- engine: xtreg (requires xtset + fe), ivregress 2sls with dedicated
  listwise deletion across depvar/exog/endog/instruments; shared
  coefficient-table renderer extracted
- fixtures: z1/z2 instrument columns (drawn after existing draws, earlier
  golden values bit-identical), manual within/2SLS algebra in R
- 75 tests green (swift test and xcodebuild with GPU suites)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

- DTAReader: formats 117/118/119, both byte orders, all numeric storage
  types widened to Float64 with Stata missing codes (., .a-.z) mapped to
  the validity mask; str# and strL (GSO) load as strings; value labels
  read past but not yet applied; pre-117 files rejected with a clear
  message
- DTAWriter: format 118 (UTF-8, LSF), doubles + str# up to 2045 bytes,
  real map offsets so Stata/haven/pandas can seek
- ZQDataStore routes .dta through the native path for use and save
- ZQColumnData equality now ignores value slots at missing positions
  (undefined by contract, often NaN — synthesized == failed on NaN != NaN)
- Fixtures: haven-written 117 + 118 files; tests assert bit-identical
  numeric loads vs the CSV, lossless write/read roundtrip; haven
  cross-reads Metrika-written files (verified)
- 61 tests green

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>