fix(datagen): return errors instead of panicking on developer-error input (PIPE-1003)

[Dylan-M]

Stacked on #275

Proposed Change

Converts datagen's panic-on-developer-error paths to error returns. Blitz is
embeddable, so a panic on recoverable input takes down the host process; these
paths now return an error the host can handle.

• GenerateSystemIdentity, GenerateEnvironment, and the group and username paths
  return an error on nil pointers and invalid input rather than panicking.
• Adds an error-returning parseCIDRs primitive; the panicking mustParseCIDRs
  form stays only for parsing compile-time constants.
• Callers and tests handle the returned errors, and the former panic-expecting
  tests now assert error returns.

Validation: the change has no user-facing surface, so it's exercised through unit
tests. Each converted path returns an error (nil domain, invalid CIDR) rather than
panicking, and the happy paths stay clean.

Checklist

• Changes are tested
• CI has passed

[Dylan-M]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(config): reject invalid network CIDRs at environment load (PIPE-1002) #281
• feat(generators): emit simulated host identity on every record (PIPE-1036) #280
• feat(resource): support list-valued resource attributes (PIPE-1253) #279
• feat(datagen): environment foundations for simulated host identity (PIPE-1036) #278
• refactor(datagen): thread an injected logger, drop global-logger use (PIPE-1067) #277
• fix(datagen): return errors instead of panicking on developer-error input (PIPE-1003) #276 👈 (View in Graphite)
• feat(datagen): add IPv6 to NetworkIdentity and public emission (PIPE-1001) #275
• feat(datagen): add storage, network, and appliance-OS identities (PIPE-1035) #274
• main

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