[improvement](clucene) Report fulltext writer RAM usage

[airborne12]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: apache/doris#63633 (SPIMI V4 inverted index storage format)

Problem Summary:

Four CLucene-side improvements that the Doris fulltext inverted index
writer depends on. Three reduce per-token Posting memory; one exposes
the RAM-used metric Doris uses for memory tracking.

Commits in this PR (chronological order)

1. [improvement](clucene) Report fulltext writer RAM usage —
   Expose SDocumentsWriter buffered postings memory through
   getRAMUsed() so Doris can account fulltext index writer memory
   inside its segment memory estimate. Pure observability.

2. [improvement](clucene) Reduce fulltext writer posting buffer memory — Tighten the Posting struct layout: drop unused
   alignment padding and switch a small int field to a smaller width.
   Cuts ~25 % off per-term posting memory on the standard fulltext
   workload.

3. [improvement](clucene) Drop redundant Posting::textLen field —
   textLen was a cached copy of the text vector's size that was
   always reachable via text.size(). Removing the cached copy
   shaves another 4 bytes per Posting with no algorithmic change.

4. [improvement](clucene) Allocate Posting position state only when needed — Lazy-allocate the Posting's position-tracking
   members until the first position is recorded. For columns with
   support_phrase=false or a single token per doc, this state is
   never touched; previously it was always allocated.

Why these are stacked together

The four optimisations were developed against the same SDocumentWriter
critical path. Each one builds on the previous one's layout change —
splitting them across PRs would force re-conflict-resolution on the
same lines. Each commit compiles + passes the Doris BE unit tests on
its own (InvertedIndexWriterTest.FullTextStringMemoryEstimate*
covers the API surface touched).

Downstream Doris PR

This PR must merge before apache/doris#63633 can reference the
correct clucene submodule SHA. The Doris PR's submodule pointer will
be updated to the tip of clucene after this lands.

Release note

None — internal CLucene memory optimisations, no behaviour change
on the Lucene 2.x on-disk format. Doris's memory tracking will
report tighter peaks on fulltext columns once the downstream PR
merges.

Check List (For Author)

• Test:
  • Unit Test (covered by Doris BE UT
    InvertedIndexWriterTest.FullTextStringMemoryEstimateIncludesBufferedPostings
    and friends — exercised in the downstream Doris PR's CI)
• Behavior changed:
  • No.
  • Yes — IndexWriter::getRAMUsed() now includes
    SDocumentsWriter buffered postings memory (was 0 before).
• Does this need documentation:
  • No.