fix(character): resolve default rolling method from config, not "standard"

[QMalcolm]

Closes #123

Why

roll_generated_value fell back to Map.get(rolling_methods, method_id || "standard") — domain vocabulary from dnd_5e_srd's TOML hardcoded in library source, violating the library/config boundary. A second system without a method named "standard" crashed with nil.dice for any methodless generated node; an unknown explicit method id crashed the same way.

What

The config mechanism already existed: rolling methods carry a default flag (dnd's standard sets default = true), parsed by the loader and then ignored by the library. Now:

• Methodless generated nodes resolve to the method flagged default = true.
• Unknown method id → clear raise naming the id; methodless node with no default → clear raise explaining the fix. No more nil.dice.

Load-time validation added (warn-and-degrade, matching loader posture):

• The missing-method warning moved to a post-load pass — a methodless node is now legitimate when a default exists (possibly in another concept file), so the check needs the complete rolling_methods map. It fires only when no default is declared.
• Unknown method reference on a generated node warns.
• drop values other than "lowest" warn and are ignored (previously silent).
• Multiple default = true methods warn as ambiguous.

The "rolling_method" reserved concept type and its keys (name/dice/drop/default) are now documented in the structural-vocabulary moduledoc; it was special-cased in code but missing from the reserved-types list. (Vocabulary constant centralization stays #125.)

Verification

• grep '"standard"' apps/ex_ttrpg_dev/lib/ — zero hits.
• 5 new/updated loader tests (no-default warning, default-satisfies-methodless, unknown method, unsupported drop, ambiguous defaults) + 3 new character tests (default-method roll works, unknown-method raise, no-default raise).
• dnd_5e_srd still loads with zero warnings and generation behaves identically (existing tests unchanged).
• Full suite (312 + 55), credo, dialyzer, CodeScene delta all pass.

Summary by Bito

• Introduced a 'rolling_method' concept type to define character generation dice rolling rules, including support for defaults and drop-lowest mechanics.
• Updated character generation to automatically resolve rolling methods, defaulting to the system-wide default if no method is specified.
• Added validation and warning logic in the rule loader to detect ambiguous defaults, unsupported drop values, and missing rolling methods.
• Improved error handling for unknown rolling methods during character generation.