feat(fuzz): coverage-guided fuzz testing pipeline with cargo-fuzz (#460)

* feat(fuzz): add cargo-fuzz crate with 4 coverage-guided targets

- subscription: full lifecycle fuzzing (create_plan → subscribe →
  charge → pause → resume → cancel)
- pricing: boundary price values, refund math, charge timing
- rate_limit: per-function rate-limit enforcement windows
- state_machine: illegal state transitions (double-cancel,
  charge-while-paused, etc.)
- utils: shared helpers for byte parsing, env setup, time management
- .gitignore for cargo-fuzz artifacts and corpus directories
- seed corpus files for all 4 targets with edge-case inputs

Each target parses raw bytes from libFuzzer into a command stream
and catches panics to keep fuzzing on invalid inputs.

* feat(ci): rewrite fuzz CI with cargo-fuzz matrix, corpus cache, crash triage

- 4-target matrix (subscription, pricing, rate_limit, state_machine)
  running in parallel
- nightly toolchain + cargo-fuzz installation
- seed corpus copied from .github/corpus/ and cached between runs
- 30-minute run per target with AddressSanitizer
- crash artifacts uploaded on failure with 14-day retention
- corpus uploaded weekly for coverage continuity
- scheduled weekly run (Monday 06:00 UTC)
- explicit error notification with local reproduction command

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Co-authored-by: Kefas Kingsley <kefasiceking@gmail.com>

Author: activatedkc

.github/corpus/pricing/max_price

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.github/corpus/pricing/min_price

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