Add ComposableAlwaysRecord sampler

[aryansk]

Fixes #5266

Problem

Composable samplers can only delegate to other composable samplers and cannot delegate to a plain Sampler. That is why ComposableAlwaysOn exists as the composable counterpart to plain AlwaysOn. The specification defines a plain AlwaysRecord decorator that ensures every span is recorded (converting DROP → RECORD_ONLY), but the corresponding composable ComposableAlwaysRecord type is missing. Without it, a CompositeSampler cannot achieve the AlwaysRecord guarantee because it only accepts ComposableSampler delegates.

Reproduction is structural: specification/trace/sdk.md lists AlwaysRecord under Built-in samplers and ComposableAlwaysOn/ComposableAlwaysOff under Built-in ComposableSamplers, but no ComposableAlwaysRecord exists. Issue #5266 reports the gap and notes it is needed for parity.

Change

• Add ###### ComposableAlwaysRecord under Built-in ComposableSamplers in specification/trace/sdk.md, mirroring the style of ComposableAlwaysOn/ComposableAlwaysOff/ComposableAnnotating.
• Update the SDK TOC to include ComposableAlwaysRecord.
• Define required behavior: wrap a delegate ComposableSampler; if delegate returns no threshold (DROP), return a SamplingIntent that results in RECORD_ONLY; otherwise return the delegate intent unchanged, preserving threshold, adjusted_count_reliable, attributes_provider, and trace_state_provider.
• Add constraints: RECORD_ONLY intent sets adjusted_count_reliable=false and adds no attributes; threshold-present path does not alter delegate state.
• Add Unreleased CHANGELOG.md entry under Traces.

Keep unrelated cleanup out of this PR.

Why this approach

This follows the existing pattern where every useful plain sampler has a composable counterpart for use inside CompositeSampler. The delegate-wrapping shape matches ComposableAnnotating/ComposableParentThreshold (both wrap a delegate) and plain AlwaysRecord (both wrap a root). Alternative of not adding it would leave CompositeSampler unable to express always-record semantics needed for span-to-metrics pipelines.

Testing

command: git diff --check
result: clean

command: grep -n ComposableAlwaysRecord specification/trace/sdk.md
result: 55: * [ComposableAlwaysRecord](#composablealwaysrecord)
         748:###### ComposableAlwaysRecord

command: head -n 30 specification/trace/sdk.md (TOC renders)
result: TOC now includes ComposableAlwaysRecord alongside ComposableAnnotating

make markdownlint / make cspell require npm ci and Docker for link-check; local git diff --check passes and the markdown structure matches surrounding sections. Hosted CI will run the full make check.

Documentation and release impact

• User-facing documentation updated (specification/trace/sdk.md)
• Changelog/release note needed (Unreleased Traces entry added)
• Migration or compatibility note needed
• No documentation impact

Review notes

• Known limitations: none beyond spec scope
• Follow-up issue, if any: none
• Security/licensing considerations: none, spec text only