feat: typed, descriptive errors for mapping misuse and cardinality violations

[jeswr]

What this does

Adds typed, descriptive errors for mapping misuse and cardinality violations, replacing the bare throw new Error // TODO: Describe statements and the two untyped string Errors in RequiredFrom (resolving its // TODO: Expose standard errors). Consumers can now catch and discriminate these failures programmatically instead of matching on message strings.

Two new error classes, exported from mod.ts:

• MappingArgumentError (extends WrapperError) — thrown when a mapping function is invoked with an undefined termAs/termFrom mapper (e.g. a misspelt LiteralAs member in plain JavaScript). Carries the offending parameter name in argument. Now thrown by RequiredFrom.subjectPredicate, OptionalFrom.subjectPredicate, OptionalAs.object, SetFrom.subjectPredicate and Mapping.languageDictionary.
• CardinalityError (extends TermError, so rooted at WrapperError) — thrown by RequiredFrom.subjectPredicate when a singular mapping finds no value or more than one value. Carries the subject via TermError.term, the predicate IRI, and a found: "none" | "multiple" discriminator. The existing message texts are preserved verbatim, so this is not a breaking change for anyone matching on messages.

Both classes carry the repo-style dense JSDoc (summary, @remarks, titled @example blocks pairing Turtle with a mapping class, @see), and the five throwing mapping functions gained @param/@returns/@throws documentation. npx typedoc passes with no new warnings.

Tests

• Upgrades the existing assert.throws(() => ...) assertions in test/unit/term_wrapper.test.ts to assert the specific CardinalityError class and its found/predicate/term properties (removing its // TODO: Test for specific errors and // TODO: check for typed error singular no value comments).
• New test/unit/mapping_errors.test.ts covering MappingArgumentError (including the argument name) from all five entry points. Both new files are at 100% line/branch coverage; full suite passes (129 tests, 0 fail).

Issues

• Addresses Improvement: Robust code (defensive) #6 (defensive/robust code): mapping misuse and cardinality violations now fail with typed, descriptive errors.
• Addresses the error-handling half of Improvement: More arities #8, per @jeswr's comment there pointing at the error logic of fix: error on undefined singular #13 (whose no-value message this preserves). The additional-arities half of Improvement: More arities #8 is intentionally not part of this PR.

Relation to #61 / #73 / #74

Supersedes no part of them: all three leave these TODO-marked bare throws untouched (#61 keeps the throw new Error // TODO: Describe lines as-is in its rewrite of the same functions). Merge-order conflicts with #61 in these five files would be textual and small.

Review timing: This draft was prepared with Claude; I (@jeswr) will personally review it before it progresses. I'm currently batching a lot of work in flight, so expect active review Wednesday-Friday (8-10 July).