vm: count derivation string-coercion against max-call-depth

[dan-v]

Problem

A self-referential list or attrset reaching a derivation argument recurses natively through the string coercion until the fiber stack faults: SIGSEGV, where Nix reports stack overflow; max-call-depth exceeded.

$ fix eval --read-write-mode -E 'let r = [ r ];
    in (derivation { name = "d"; system = "x86_64-linux"; builder = ":"; attr = r; }).drvPath'
Segmentation fault

Fix

Mirror Nix's mechanism: EvalState::coerceToString counts each coercion level against the same max-call-depth budget as function calls. The coercion here checks policy.max_call_depth (the max-call-depth setting, default 10000 as in Nix) once per level and raises the shared error. The forceThunkImpl native-stack soft guard backstops the recursion the same way it backstops deep thunk forcing, so an undersized stack degrades to a graceful error rather than a fault.

Verification

• Boundary parity with nix-instantiate: nesting depths 5000 and 9000 evaluate to identical drvPaths on both evaluators, 11000 errors on both, and the cycle errors identically.
• Unit tests cover deep-finite, cyclic-list, and cyclic-outPath vectors; the test binary segfaults on the parent commit.
• Full e2e and unit suites otherwise unchanged.