Add src/period-diff.ts: pure period-vs-period difference engine. Totals come
from the shared buildPeriodData rules; contributions per project and per model
carry status new/gone/up/down/flat with pct=null at A=0 (labeled New, never an
infinite percentage); normalized views per day and per 100 calls print their
denominators; day arithmetic walks local midnights so DST and month boundaries
count calendar days. historyBasis cross-checks the durable daily cache and
reports aggregate-only carried days separately instead of force-reconciling
them into the totals. diffSessions joins the drill-down on the canonical
session identity (provider + project path + sessionId), attributing the
model's own slice for the model lens.
New command: compare-periods --format json|sessions with explicit
--from-a/--to-a/--from-b/--to-b (local YYYY-MM-DD, validated loudly) defaulting
to the last seven complete days vs the seven before, plus --provider and the
existing --project/--exclude filtering. Range slicing reuses parseAllSessions'
call-granularity behavior, so a session straddling the boundary contributes to
each side by its own calls.
Tests: engine fixtures including the X/Y/Z acceptance reconciliation, A=0/B=0,
unequal lengths, DST and month walks, carried history, a real-parse boundary
session split by call and joined back into one drill-down row, and CLI runs on
a temp HOME fixture.
Drop pin-baselines.mjs: pinning is a once-per-phase paste and the
script hard-coded this phase's iteration-log row, so every later phase
would edit by hand anyway. Replace three hand-rolled argv parsers with
node:util parseArgs. Read machine facts from node:os instead of eight
subprocess probes (hw.model keeps its sysctl call). Track fixture bytes
with a running counter instead of re-walking the tree per file. Delete
the hover proxy that measured an argv the dock never spawns, plus
unused exports (ensureDir, assertAbsolute, GENERATED_FIXTURE_DIR).
Fixture output is byte-identical for the same seed (verified by full
tree diff against the pre-refactor generator) and all harness metrics
reproduce within run-to-run noise.
Scripted isolated-HOME measurements for session parse, incremental
append, period switch, CLI cold start, and RSS. Extends the #1164
timings.csv shape without touching parser, cache, or UI hot paths.
