chore: implement phase 2 - #2780

* chore: implement phase 1

Strict parsers for the state and ruler endpoints (H1), a Prometheus-style
duration parser, and fixtures sliced from real Grafana 13.1.0 payloads
covering every required/optional-field and must-error case, including the
"Normal (NoData)"/"Normal (Error)" composite reason states found live in the
current fleet capture (not in the original plan's vocabulary).

* chore: apply code review comments

* chore: implement phase 2 (#2780)

* chore: implement phase 2

Fix retry-error conflation, measure full poll latency, and harden Source test doubles for concurrency.

* chore: enhance unit tests

* chore: address code review comments

* chore: implement phase 3 (#2781)

* chore: implement phase 3

Add Resolve() for alert name resolution (uid:/Title/Folder/Title/
Folder/Group/Title forms, UID collapse, no-match suggestions) and the
grafana-alertcheck CLI's list subcommand, the first runnable piece of
the gate.

Incorporates review fixes: reject empty path segments in classifyForm,
guard uid: against an empty suffix, scope the no-match rule count and
suggestions to supported rule kinds only, and exit 0 on -h/--help.

* chore: enhance unit tests

* chore: implement phase 4 (#2782)

* chore: implement phase 4

Add per-rule poll timings, scheduler, and budget check (P4).

- schedule.go: DeriveTimings, Scheduler, CheckBudget (§5)
- Address review: add Folder/Title resolve test, rename
  CheckBudget's minPollEvery to tightestUID

* chore: enhance unit tests

* chore: address code review comments

* chore: implement phase 5 (#2783)

* chore: implement phase 5

Add the JSONL evidence log (P5).

- log.go: Header/Poll records, reduction, H2 transition markers,
  §3.2 verification, append-only Writer with flock, ReadLog
- flock_unix.go: non-blocking exclusive lock, unix only
- schedule.go: DeriveTimingsFromLog — log-mode cadence comes from the
  header, never from the definitions

* chore: remove unix build tags

* chore: address code review comments

* chore: implement phase 6 (#2784)

* chore: implement phase 6

Invariant defended: H2. The one question: can watch return success over a
window that nothing is recording?

Watch() records the first observation of each non-skipped rule, then detaches
a child that polls at the cadence in the header. The parent returns only after
the child reports ready on an inherited pipe, and writes the pidfile after
that. A clean stop writes the sentinel; a hard error does not.

* chore: add a unit test, remove build tags

* chore: address code review comments

* chore: implement phase 7 (#2785)

* chore: implement phase 7

Invariant defended: H3. The one question: can a rule be called alive
because it looked alive one poll ago?

proveCoverage (grafana-alertcheck/internal/gate/coverage.go) is the pure
coverage function: nine checks over one rule's polls — sentinel,
from-bounds, heartbeat continuity, health error/nodata, liveness,
in-window pause, rule absence, KeepLast. Liveness is absolute, never a
delta. Cross-domain comparisons translate by each poll's own skew and
widen boundary segments by its skew bound, fail-closed.

* chore: enhance unit tests

* chore: address code review comments

* chore: implement phase 8 (#2786)

* chore: implement phase 8

Invariant defended: H6/H7. The one question: can a violation ever
outrank an unobservable rule, or can a pass happen without
Violations empty and err nil?

Adds classify.go: the pure per-instance classifier (outcome table,
preexisting policy, BadFor) and decide(), the seam combining
proveCoverage with those timelines under one Policy. Consolidates
rule-poll filtering and skew translation onto pollsForRule/runnerTime,
shared with coverage.go.

* chore: rename some vars + add unit tests

* chore: address code review comments

* chore: implement phase 9 (#2787)

* chore: implement phase 9

Invariant defended: H5/H7. The one question: can check report a pass
over a window it did not prove?

Check() is the I/O shell around the pure decide(). Single-step
synthesizes the header and its own sentinel, so no mode flag reaches
the pure layer. Log mode stops the recorder before the one full read.

The header, not a definition re-resolved after the window closed, is
the authority for what was paused when the window opened — it decides
`skipped`, the drain set, and the transitionGrace max. The flock, not
the pidfile, is the authority for whether a writer still exists.

* chore: remove unix build tag

* chore: implement phase 10 (#2788)

* Wire watch/check subcommands to the gate library, with a table+JSON
renderer and H6/H7 exit-code mapping. Extend Result with per-rule/global
thresholds and a real skew bound; export SkewHardLimit; reject --states
normal.

* chore: fix goreleaser.yaml and add version command

* chore: implement phase 11 (#2789)

* chore: implement phase 11

Add coverage.go's declared-KeepLast check (no_data_state/exec_err_state,
not just an observed reason) and close the remaining §22 gaps: newly_bad's
no-early-exit clock assertion, a recorder-mode gap right after the deploy,
a rule's own coverage gap overriding its own recovery, a genuinely
skew-discriminating staleness test, exit-2 consequences on two
Reason-only coverage tests, and end-to-end checks for log-name
collapse, a truncated log, and the real watch-written state histogram.

* chore: address code review comments

* chore: more concise comments (#2790)

* chore: more concise comments

* fix: merge conflict

* chore: shorten comments

* fix: resolve conflict

* chore: use testify's require in tests (#2792)

* chore: use testify's require in tests

* chore: move remaining assumptions to testify

* chore: address code review comments

* chore: fix logging and std out printing (#2798)

* chore: fix logging and std out printing

* chore: truncate to seconds when comparing from time

* chore: add centralized docs (#2802)

* chore: add centralized docs

* chore: further update docs

* chore: address code review comments (#2807)

* chore: address code review comments

* chore: get rid of goreleaser