feat(datagen): environment foundations for simulated host identity (PIPE-1036)

[Dylan-M]

Stacked on #277

Proposed Change

Adds the foundations for generators to describe a simulated host from a
datagen.Environment rather than the machine blitz runs on. Nothing emits the
identity yet; this is the plumbing the rest of the stack builds on.

• An environment: config block that builds a datagen.Environment (domain, seed
  config, per-type counts).
• embed.Host.Environment, for an embedding host to supply its own environment.
• Environment.SystemForKey, a deterministic key-to-host selector.
• OS taxonomy helpers (ParseOSType, OSTypeFromGOOS, SemconvOSType), including
  accepting macos/darwin for the hostmetrics OS knob.
• StaticResources, which splits a worker's constant resource attributes from the
  per-record ones so identity is projected once and reused.

No generator behavior changes; existing output is byte-identical.

Validation: load a config with an environment: block and confirm it parses and
runs with no change to output. Tests cover the config round-trip, SystemForKey
determinism, the OS helpers, and StaticResources merge semantics.

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 👈 (View in Graphite)
• 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
• 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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