feat: assumed_count caveat for counts that rest on a data source placeholder

[andreacappelletti97]

What

A new caveat code, assumed_count. When a resource's count / for_each, or that of the module it sits in, is computed from a data source c3x gives a placeholder (data.aws_availability_zones is three zones in the provider's region, aws_region, aws_caller_identity, google_client_config), every priced instance now says so:

  aws_nat_gateway.main[0]
    ⚠ instance count assumes data.aws_availability_zones.available.names = [us-east-1a, us-east-1b, us-east-1c]; price a plan JSON for exact counts

How

• Parser: expandInstances already stripped placeholder marks from count/for_each; it now renders them (describePlaceholders) into instance.assumed. moduleEnv.assumedCount carries a module instance's assumption down, so every resource under a placeholder-counted module inherits it (joined with the resource's own, deduplicated). The result lands in the new domain.Resource.AssumedCount.
• Calculator: assumedCountCaveat turns it into a resource-level caveat, only when the resource's subtotal is non-zero: a free subnet's count can't move the total.
• Output: no renderer changes needed; resource caveats already render in text, markdown, the PR comment, JSON (code: assumed_count), count toward caveat_count, and fail --strict (exit 3). The --strict help text mentions the new case.
• Logging: the stderr WARN for a resource or module count/for_each is now DEBUG. The caveat says the same thing next to the number, in every format; the warning just printed four raw slog lines above the report (one per resource, free ones included). A dynamic block's for_each over a placeholder has no caveat, so it still warns.
• README caveat table and CHANGELOG [Unreleased] updated.

Tests

• Parser: count and for_each over the zones record AssumedCount on every instance; a literal count and no count record nothing; module count over the zones propagates to the module's resources, a literal module count doesn't. Existing placeholder tests now assert on AssumedCount instead of the log.
• Calculator: caveat with the exact detail on a priced resource; none on a free one; none without an assumption.
• CLI (flat-price test server): JSON carries assumed_count on the per-AZ instances only (not the literal-count instances, not the free VPCs), text and markdown show it for the three priced instances, --strict returns the caveat error and lists assumed_count.
• Golden files unchanged: they run --offline, where every price is $0, so no resource gets the caveat.

Demo (c3x-demo/aws, live pricing)

$ c3x estimate --path . --format json | jq '.caveat_count'
6        # aws_eip.nat[0..2], aws_nat_gateway.main[0..2]; the free subnets get none
$ c3x estimate --path . --strict; echo $?
c3x: --strict: the estimate has 6 caveat(s):
  assumed_count          6
3