Feat/codebase improvements

[Splode]

Bug fixes, library improvements, and word list quality

Bug fixes

• Seed sentinel value removed — --seed -1 previously silently fell back to random generation because -1 was used as the "no seed provided" sentinel. The flag now uses pflag.Lookup("seed").Changed to distinguish an explicit seed from an absent one, making all int64 values valid seeds.
• Spurious collision-avoidance loop removed — Generate() retried the noun index whenever it equalled the adjective index, on the assumption this prevented duplicate words. It didn't: the indices are into different-sized arrays and equal integers don't mean equal words. The loop is gone.
• Size validation is now eager — WithSize returns (GeneratorOption, error) instead of a silent no-op option. Invalid sizes fail at construction time rather than surfacing as an error on the first Generate() call.

New features

• --format / -f flag — CLI now supports --format json to emit all names as a JSON array (["name1","name2"]), useful for scripting. Default behavior is unchanged (plain).
• WithDictionary and NewCustomDictionary — Library callers can now supply custom word lists. NewCustomDictionary(adjectives, adverbs, nouns, verbs []string) builds a Dictionary with per-category overrides; nil slices fall back to the embedded defaults. Fulfills a long-standing TODO.
• --quantity validation — Passing --quantity 0 or a negative value now exits with a clear error instead of silently producing no output.

Library API

• ParseCasing added — Replaces CasingFromString as the canonical name (idiomatic Go convention for string parsing). CasingFromString is kept as a deprecated alias.

Performance

• split() in dictionary.go replaced bufio.Scanner on in-memory strings with strings.Split — simpler and avoids unnecessary reader allocation.
• applyCasing replaces a map[Casing]func lookup with a switch — zero allocation, no hash overhead.

Word list quality

• Verb tenses normalized: 169 past-tense and past-participle forms converted to 3rd-person singular present (e.g. abandoned → abandons, achieved → achieves). Net change: 448 → 419 verbs.
• Adjective/noun overlap resolved: 72 words appeared in both lists. Each was removed from the less-appropriate list (e.g. cherry, copper, ruby removed from adjectives; blue, cold, dark removed from nouns). 2-word combination space: 4.65M → 4.51M — well above the stated floor.

CI

• actions/setup-go updated v2 → v5; fixes 403 failures when downloading Go for macos-latest (ARM64) runners.
• Matrix minimum Go version aligned with go.mod: ~1.17 → ~1.19.
• Removed GO111MODULE: "on" (no-op since Go 1.16).