Fix nil-comparison issues: bracket access and cache invalidation

[gillisd]

Summary

• Flexor#[] now returns Ruby's literal nil for missing keys AND for empty Flexors already in storage, so ||=, ||, and conditional assignment behave the way Ruby developers expect. Phantom Flexors no longer leak into truthiness checks.
• Method access (f.foo) keeps autovivifying — the documented "Safe Chaining" feature (f.user.name = "Joe") is preserved end-to-end, including when interleaved with bracket access.
• After #delete or #clear, cached singleton getters from prior method access are now invalidated, so re-accessing a removed key vivifies a fresh child Flexor instead of returning a stale nil. FlexKeys invalidates the case counterpart as well, so deleting :user_name also clears any cached :userName getter.
• Asymmetric contract is intentional and documented in README + docs/specification.yaml. Cached method getters resolve through FlexKeys correctly (camelCase ↔ snake_case still round-trips on the second call).

Test plan

• bundle exec rake is fully green (377 examples, 0 failures; rubocop 0 offenses)
• Regression: f = Flexor.new; f[:foo] ||= :bar; f[:foo] → :bar
• Regression: same after a method touch — _ = f.foo; f[:foo] ||= :bar; f[:foo] → :bar
• Method-style chain still works: f = Flexor.new; f.user.name = "Joe"; f[:user][:name] → "Joe"
• FlexKeys cached read still resolves: s = Flexor.new(fooBar: "v"); s.foo_bar; s.foo_bar → "v" on both calls
• Cache invalidation on delete: f = Flexor.new(user: { name: "a" }); f.user.name; f.delete(:user); f.user.name = "b" returns "b" (no NoMethodError on nil)
• Cache invalidation on clear: same shape with f.clear substituted for f.delete(:user)
• FlexKeys case-alternate invalidation: s = Flexor.new(user_name: { nickname: "a" }); s.userName.nickname; s.delete(:user_name); s.userName.nickname = "b" returns "b"
• Documented contract change: Flexor.new[:user][:name] = "x" raises NoMethodError (use method-style chain instead)
• Frozen-store contract: bracket on missing key returns nil silently; method-style autovivification raises FrozenError

Notes

Closes issue 3ED3C15C-4051-11F1-9F71-FE6CB9572C2F in issues.rec.

Trade-off accepted: bracket-chain assignment through an unset intermediate (f[:user][:name] = "x" when :user is unset) now raises NoMethodError. Use f.user[:name] = "x" or f.user.name = "x" instead. README and contract specs document this explicitly.

[gillisd]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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