Cardano Updates

Metatheory: simplify variable representation by using Fin instead of nested Maybe

The memory_bytes field was just memory_budget × 8 (word size), providing
no new information - clients can trivially compute this themselves.

Removed from:
- EvalResult type and ToJSON/FromJSON instances
- SPEC.md schema documentation and examples
- Integration tests

Two issues caused intermittent test failures on CI:

1. Race condition in result file reading: Tests detected file existence
   before service finished writing, causing JSON parse errors
   ("Unexpected end-of-input"). Fixed by using atomic writes - write to
   a temp file first, then rename to final path.

2. Overly strict timing assertions: Very fast evaluations (especially
   in CI environments) can complete in <1ns, causing cpu_time_ns=0.
   Relaxed assertions from `> 0` to `>= 0` since 0ns is legitimate
   for trivial programs.

Two issues caused intermittent test failures on CI:

1. Race condition in result file reading: Tests detected file existence
   before service finished writing, causing JSON parse errors
   ("Unexpected end-of-input"). Fixed by using atomic writes - write to
   a temp file first, then rename to final path.

2. Overly strict timing assertions: Very fast evaluations (especially
   in CI environments) can complete in <1ns, causing cpu_time_ns=0.
   Relaxed assertions from `> 0` to `>= 0` since 0ns is legitimate
   for trivial programs.

Set LineBuffering on stdout/stderr in both the service and test suite
to prevent output interleaving when multiple threads write concurrently.
This ensures each line is written atomically.

The memory_bytes field was just memory_budget × 8 (word size), providing
no new information - clients can trivially compute this themselves.

Removed from:
- EvalResult type and ToJSON/FromJSON instances
- SPEC.md schema documentation and examples
- Integration tests

The memory_bytes field was just memory_budget × 8 (word size), providing
no new information - clients can trivially compute this themselves.

Removed from:
- EvalResult type and ToJSON/FromJSON instances
- SPEC.md schema documentation and examples
- Integration tests

Co-Authored-By: Claude Opus 4.5 <[email protected]>