Reject non-positive bank amounts and fix stale crew/interest dialogue

[dmccoystephenson]

Summary

Three small, independent bugs found during triage are addressed here.

• A negative bank deposit prints money and writes a save the game will refuse to load #149 — Negative amounts were accepted by Bank.deposit()/Bank.withdraw(). A negative deposit passed canAfford() (money >= a negative number is always true) and minted cash, while a negative withdrawal drove player.money negative — both wrote a save that schemas/player.json's "minimum": 0 constraint would reject on the next load. Non-positive amounts are now rejected with a re-prompt, mirroring the existing is None retry branch.
• Crew say the fleet is full and to buy a bigger boat while berths are still free on the other boats #152 — The crew NPC's "berths full" dialogue and its unlock condition were keyed off the best hull's maxWorkers, even though hiring has been fleet-wide (boats.totalCrewBerths) since a prior change. A player with a second boat and open berths was told to buy a bigger boat while hiring still worked normally. Both the condition and the response text are now derived from totalCrewBerths, matching the README's documented "one shared roster" rule.
• The bank teller says the more you save the more you earn, but interest is capped at $50/day and the cap is never disclosed #158 — The bank teller's "interest rates" dialogue claimed "the more you save, the more you earn," which stopped being true once MAX_INTEREST_PER_DAY was introduced to cap the daily payout. The line is now derived from INTEREST_RATE/MAX_INTEREST_PER_DAY so it states the real rule and the point at which it caps, and can't drift from the constants again.

The broader, structural write-side validation this triage also flagged (unvalidated writes letting an invalid save land on disk at all) is left to #160, which is a materially larger change touching saveFileManager.py/*JsonReaderWriter.py.

Issues deferred this cycle

The rest of the open backlog is deferred without further comment, since this cycle's scope was kept to three small, low-risk fixes: #160 (save-write atomicity/validation — large, touches protected save-path files), #156 (voyage-event test coverage — a sizeable standalone test-expansion effort), #153 (web front-end "ended" screen), #151 (overnight report dropped at three call sites), #150 (corrupt save vanishing from the menu), #143 and #142 (save/load error reporting).

Test plan

• python3 -m compileall -q src
• Full suite: 731 passed under headless SDL drivers (SDL_VIDEODRIVER=dummy, SDL_AUDIODRIVER=dummy)
• New regression tests added for negative-amount rejection (deposit/withdraw), the interest-rate dialogue's disclosed cap, and the fleet-wide berths dialogue/condition

Closes #149
Closes #152
Closes #158

This PR description was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — the diff touches only src/location/bank.py, src/npc/villagers.py, and their matching test files; every changed line belongs to one of A negative bank deposit prints money and writes a save the game will refuse to load #149/Crew say the fleet is full and to buy a bigger boat while berths are still free on the other boats #152/The bank teller says the more you save the more you earn, but interest is capped at $50/day and the cap is never disclosed #158.
• Tests-new: PASS — test_deposit_negative_amount_is_rejected, test_withdraw_negative_amount_is_rejected, test_npc_interest_dialogue_discloses_the_daily_cap, and test_createCrewNPC_crowded_question_stays_hidden_with_room_on_another_boat each exercise the new behavior.
• Tests-fix (empirical): PASS — source fixes were reverted to the origin/main state (tests left in place) and all four new tests were confirmed to FAIL (4 failed); the fix was then restored and the full suite passed (731 passed).
• Sibling structure: PASS — new tests follow the existing # prepare/# call/# check pattern used throughout test_bank.py/test_villagers.py.
• Sibling renames: N/A — no parallel identifier pair was renamed this PR.
• Docs: PASS — README.md already documents fleet-wide hiring ("Crew come from one shared roster") and says nothing about a specific interest rate/cap, so no doc update was needed; the code/dialogue now matches what was already documented.
• Issue resolution: PASS — A negative bank deposit prints money and writes a save the game will refuse to load #149 (deposit/withdraw now reject amounts <= 0), Crew say the fleet is full and to buy a bigger boat while berths are still free on the other boats #152 (crowded dialogue + condition now use totalCrewBerths), The bank teller says the more you save the more you earn, but interest is capped at $50/day and the cap is never disclosed #158 (interest dialogue now states the actual rate and cap) are each fully addressed by the diff, not partially.
• CI: PASS — gh pr checks 167 is green (test pass, 32s).
• Schema-sync: N/A — no persisted field was added or changed.
• Money-format: PASS — the new prompts use the existing %.2f formatting already used elsewhere in bank.py.
• Deterministic-tests: PASS — no new test depends on unseeded random.
• Headless-pygame: N/A — no pygame-touching code was changed.
• camelCase: PASS — _INTEREST_CAP_BALANCE is a module-level constant (matching the existing INTEREST_RATE/MAX_INTEREST_PER_DAY ALL_CAPS convention in timeService.py), and all new methods/locals follow camelCase.

Summary: three small, independent bug fixes (bank negative-amount exploit, stale crowded-crew dialogue, stale interest-rate dialogue), each with a regression test proven to fail before the fix and pass after.

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drafted by Claude on behalf of Daniel Stephenson