Tightens the hot path that converts FrameBlocks of arbitrary schema
into MatrixBlocks, with a defensive fallback for malformed cells.

- DoubleParser.parseFloatingPointLiteral: replace the >= 'I' / >= 'a'
  character guards with a single 0-9 range check on the last char.
  The previous guards over-matched and pushed too many strings into
  the slow Double.parseDouble path
- DoubleArray.parseDouble: stop wrapping the parse failure as a
  DMLRuntimeException so callers can distinguish format errors
- MatrixBlockFromFrame:
    - turn the interface into a class with a private constructor so
      Jacoco can measure it cleanly
    - on NumberFormatException / DMLRuntimeException during a bulk
      block convert, log once and fall back to convertSafeCast which
      writes NaN per offending cell instead of failing the whole job
    - add convertSafeCast / convertBlockSafeCast helpers

Frame-to-matrix conversion silently fell back to writing NaN for cells
that cannot be cast to double, logging only a single warning. This
changed behavior for callers that previously failed fast on incompatible
data. Make the lenient behavior opt-in.

- Add sysds.frame.tomatrix.warncast (default false): when false the
  conversion fails fast on number format errors; when true it warns once
  and writes NaN for the incompatible cells
- Read the flag once on the calling thread and pass it down, since the
  thread-local config is not visible to pool workers
- Extract convertStrict to share the contiguous/generic dispatch between
  the strict and warn-only paths
- Add tests for the warn-only fallback, the strict fail-fast default, and
  the tightened DoubleParser trailing-character guard