[v0.4.x] Add exponential distribution to the native backend (legacy parity)

[DougManuel]

Part of the post-v0.4.0 development plan (development/post-v040-development-plan.md, Phase 1, Task 4).

Context — this is a parity gap, not a new feature

• The legacy generator supports exponential: R/create_con_var.R:279-281 (rexp(n_valid, rate = rate_val)), documented in advanced-topics and the metadata schema (rate column).
• The native v0.4 backend stops: R/mock_spec_native.R:202-208 ("Native backend does not yet support continuous distribution 'exponential'...") — so exponential metadata forces the v0.4→legacy fallback (which also switches RNG streams, Follow-ups from PR #32 review: loader validation, strict-mode semantics, doc polish #33 item 11).
• The plumbing already exists: the recodeflow adapter stores rate on the spec variable (R/mock_spec_recodeflow.R:365), and .new_mock_spec_variable() forwards extra fields via ....

Scope

1. Add rate = NA_real_ parameter to mock_continuous() and mock_spec_continuous(), forwarded to .new_mock_spec_variable().
2. Validator rule in .validate_mock_spec_variable() (next to the existing normal-distribution block at R/mock_spec.R:792-798): distribution == "exponential" requires rate > 0.
3. .generate_native_continuous() branch using inverse-CDF truncation to the declared range (qexp(runif(n, pexp(lo, rate), pexp(hi, rate)), rate)), mirroring .native_truncated_normal().
4. Document the intentional semantic difference: legacy rexp() is unbounded; native truncates to the declared range, consistent with the native normal. NEWS entry.

Out of scope: simstudy-backend exponential mapping; distribution registry (see plan, deferred register).

Acceptance

• TDD; values within range; seed-reproducible; validator message tested.
• Full suite green. Enables the exponential half of Add success-path tests for normal and exponential continuous distributions #24's success-path tests on the native path.

Relates to #24.