docs: write the invariants the removed notes file held into docs/

[sshaplygin]

When the working-notes file stopped being tracked (#9), the invariants it
recorded were meant to move into docs/. Stage A4 (#13) was closed without
doing that. An audit of twelve load-bearing rules against docs/ found six
with no statement there at all — not the three first reported.

Invariant	Now in
Every mutation of a policy happens while it is not the active one (switchLocked's ordering rule)	design.md
A gradual window has three roles, not two — the source is neither active nor a shadow	design.md
Sampled counts are never scaled back up by 1/rate	design.md
A selection naming Undefined, or a policy the cache does not hold, means no change (previously only a comment inside the illustrative adapter skeleton)	design.md
Advice: Epochs counts reporting epochs, not ticks; measurements start over on a role change; ties break by policy type	advisor-mode.md
The order of Keys() is not portable: 2Q returns frequent-then-recent, ARC recent-then-frequent	policies.md

The ordering claims were checked against golang-lru's source: 2Q's Keys
appends its recent list to its frequent one, ARC's appends t2 to t1.

Stale names left by #14

The three-phase epoch left the old function name in four places:
docs/design.md's architecture sketch still read
runEpoch -> selectPolicyLocked, and stability.go plus one test comment
referred to a function that no longer exists. That test comment also said the
parked epoch goroutine held the cache mutex; it now holds banditMu and no
cache lock.

Not in this change

AdaptedCache.Keys is documented as "oldest first", which the table above
shows is false for 2Q and ARC, and enforceCapacityLocked removes keys[0] on
that assumption. That is a code fix with a test, and it belongs to the next
stage (the Keys() order canary), not to a documentation change.

Verification

internal links in docs/ and README.md   49 checked, 0 broken (fenced code excluded)
gofmt / go vet                          clean
go test -race -count=3 (touched tests)  ok
make all                                Release checks passed