refactor(planning): consolidate plan observations in one module

Move time-off periods, work sequences, public-holiday weeks, balance
observations, the placement list and both tie keys into
plan-observations.ts. The code moves without behavior changes; all seven
benchmark scenarios produce byte-identical outcomes, explanations and final
states, and the frozen-engine check passes. Benchmark timings stay within
noise.

Refs #282

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/benchmark/issue-279.md

@@ -0,0 +1,47 @@
+# Issue #279: incremental exact scoring
+
+Step-by-step record for [issue #279](https://gitea.nessar.fr/nessar/vac-optimizer/issues/279).
+Each step keeps planning outcomes, stable explanations and final states identical
+to develop after PR #278 (`a871f8f`). The
+[frozen-engine differential check](README.md#frozen-engine-differential-check)
+guards every step.
+
+All measurements use Node 24.18.0, Bun 1.3.14 and Vitest 5.0.3 on the development
+laptop, with no other test, build or profiling workload running alongside the
+protocol. Only same-session before/after pairs are comparable.
+
+```sh
+NODE_OPTIONS=--max-old-space-size=2048 timeout 300s bunx vitest run src/app/benchmark/leave-planner-benchmark.spec.ts --maxWorkers=1
+```
+
+The per-scenario figures below are the means of the two measured calculations.
+
+## Baseline profile
+
+A CPU profile of the bundled core over all seven scenarios, with
+`calculationDate: '2024-01-01'`, put 50.5 of 67.0 loaded-year seconds in
+candidate evaluation (`evaluateRuleCandidate`). Of those, 41.3 seconds went to
+active-objective evaluation: 15.6 seconds in time-off-period observations,
+11.5 seconds in building observations and 7.6 seconds in sorting placement dates
+for the placement tie key and the observation cache key. The asynchronous
+adapter's 1 ms timer yields cost about 7% on typed obligations.
+
+## #282: consolidate plan observations
+
+Time-off periods, work sequences, public-holiday weeks, balance observations,
+the placement list and both tie keys now live in
+`src/app/services/plan-observations.ts`. The code moved without behavior
+changes. All seven scenarios produce byte-identical outcomes, explanations and
+final states compared with `a871f8f`.
+
+| Measurement         |    Before |     After |
+| ------------------- | --------: | --------: |
+| Vitest duration     | 262.000 s | 262.030 s |
+| Loaded year         |  66.010 s |  66.497 s |
+| Typed obligations   |   4.887 s |   4.964 s |
+| Two years           |   2.914 s |   2.977 s |
+| Three years         |  10.203 s |  10.049 s |
+| Representative year |   0.557 s |   0.569 s |
+
+The differences are within run-to-run noise. The functional suite passes
+666 tests across 34 files, including the frozen-engine check.