refactor(server): split dispatch into handler modules and standardize error formatting

[QMalcolm]

Closes #132.

What

Four commits, mapping to the issue's four fixes:

1. Sub-choice validation into the library — fetch_choice_def! and valid_sub_choices move from Serializer to Characters.Advancement (with Characters wrappers), following refactor: move award application and progression choice resolution into the library #131's pattern. Their shared option-derivation helper becomes public Characters.sub_choice_options/2 and the Serializer's rendering path now uses it, so the options shown and the options validated can no longer drift. The Serializer moduledoc now states its actual responsibility instead of apologizing for its existence.

2. One error contract, one formatter — handlers return {:ok, data, state} | {:error, message}; handle_command builds protocol envelopes in exactly one place. All domain {:error, reason} terms render through the new Server.Errors.message/1 (awards, progression choices, preparation/activation, inventory-field validation). The characters.spells, inventory.add, and inventory.set handlers no longer leak inspect(reason) terms to the Rust frontend. Same-condition-same-message unifications: :value_must_be_integer, :missing_effect_target, and the two missing-pending-character messages (characters.save now uses fetch_pending!).

3. Per-group handler modules + registry — server.ex drops from 660 lines / 26 clauses to 138 lines of dispatch: loop, rescue boundary, envelopes, and an explicit command → module registry (Handlers.Dice, .Systems, .Characters, .Build, .Inventory; shared helpers in Server.Common). Both resolve_choice forms now sit adjacent in Handlers.Characters (the scoped one previously lived under the Inventory section, 227 lines from its sibling). Known commands with malformed arguments now get "invalid arguments for command: ..." instead of falling through to a misleading "unknown command".

4. Complete protocol docs — all 22 registry commands documented with examples (previously six were missing: build_start/select/resolve_sub/finish, random_resolve, delete), grouped to mirror the handler modules, with a builder-flow walkthrough and the protocol-wide display_mode field noted. Verified programmatically that every registry command appears in the moduledoc.

Notes

• No test asserted on any error-message text, so the standardization required no test updates; all 382 tests pass unchanged.
• Errors.message/1 keeps a defensive inspect fallback clause for reasons not yet given a message — every documented library reason has an explicit clause.
• credo and dialyzer clean; codescene passes on each commit.

Summary by Bito

• Added new advancement sub-choice logic to handle concept-based choices and exclusion pools.
• Implemented `fetch_choice_def!`, `valid_sub_choices`, and `sub_choice_options` in `ExTTRPGDev.Characters.Advancement`.
• Updated CLI server handlers to support new advancement choice validation and retrieval.
• Refactored CLI server handlers to use the new advancement logic for character progression.