fix: refine siege occupation and battle pacing

[prayujt]

Outcome

Separates siege staging from settlement occupation and makes combat resolve in faster, size-proportional rounds. Conquering armies fight from beside a settlement, enter its center after victory, and must physically hold that center before ownership transfers.

Conquest changes

• Keeps initial conquest routing on the cheapest traversable tile adjacent to the center.
• After all siege defenders are defeated, updates the same conquest order to route into the center.
• Starts the 30-second occupation timer only after the army reaches the center.
• Clears an active occupation timestamp if the army is no longer at its occupation destination.
• Streams and persists the center movement transition so clients see the authoritative route and occupation phase.

Battle pacing and casualties

• Changes the authoritative battle round interval from five seconds to three seconds.
• Retains the proportional attack-power model: absolute losses increase with force size rather than using a fixed casualty count.
• Preserves fractional casualty carry so small battles are not forced to lose at least one unit every round.
• Adds a regression example where symmetric 15-v-15 soldiers lose 1 per side in the first round and 150-v-150 lose 10 per side.
• Re-arms the next round only after the current round finishes, so delayed actor work still cannot create catch-up bursts.

Companion UI

The battle countdown and progress timing are updated in the companion web PR.

Verification

• Regression tests for adjacent siege staging, center entry, center-only occupation, and occupation interruption
• Regression tests for three-second cadence and force-size casualty scaling
• go test ./...
• go build ./...
• go vet ./...
• buf lint
• goose -dir db/migrations validate
• git diff --check