Make shadow limit configurable with bounds

[ArBin1020]

Address the inherent 256MB capacity limitation on shadow FDs

• CLI Configurability: Introduced the --shadow-limit option. The default remains 256MB to preserve existing behavior, but users can now raise it for specific workloads.
• Host OOM Prevention: Enforced a hard upper bound (MAX_SHADOW_LIMIT = 512MB). Inputs exceeding this bound are automatically clamped to 512MB.
• Type Safety for Parsing: Migrated the internal limit representation to uint64_t to prevent integer wraparound and overflow when handling custom CLI inputs.
• Input Validation: Clamp negative inputs to MAX_SHADOW_LIMIT, and reject invalid or >64-bit strings with an error.

Change-Id: I814b9edb6b17b0cf3762cc95938410ecbb577f8c

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Summary by cubic

Add a configurable shadow FD size limit via the new --shadow-limit flag. Default stays 256MB; values above the 512MB hard cap are clamped to prevent host OOM.

• New Features
  • --shadow-limit BYTES to raise the shadow FD cap (0 keeps default).
  • Hard max of 512MB; higher inputs are clamped. Invalid values are rejected.
  • Limits use uint64_t for safe parsing and comparisons.

^{Written for commit 0000edd. Summary will update on new commits.}

[cubic-dev-ai]

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