[Documentation] Cosmos: Optimization Plan for PPCB High Memory Utilization (#4419)

### What this PR does

Documentation-only port. Adds a single new file:

- `sdk/cosmos/azure_data_cosmos_benchmarks/docs/PPCB_MEMORY_ANALYSIS.md`
(+1,571 lines)

No source, manifest, test, or CI changes. No behavioral impact on any
shipped crate.

### Why

The Per-Partition Circuit Breaker (PPCB) subsystem in
`azure_data_cosmos_driver` carries lazy routing state whose worst-case
footprint scales with `partition_count × failed_regions_per_partition`.
Large-container customers (~80k physical partitions) have asked us for
concrete memory-cost guidance. This document is the consolidated answer,
captured under `azure_data_cosmos_benchmarks/docs` so it lives next to
the harness that produced the numbers.

It also documents an already-applied v2 optimization
(`SmallVec<[CosmosEndpoint; 4]>` with the `union` feature replacing
`HashSet<CosmosEndpoint>`, plus `CompactString` for
`PartitionKeyRangeId`) and validates it against both a synthesized
harness and a real Cosmos DB INT account.

### Document structure

| Section | Content |
|---|---|
| §1–§9 | Methodology, harness, workload, headline numbers,
per-allocation decomposition, per-entry cost model for the v1
(pre-optimization) driver. |
| §10–§14 | Findings, recommendations, risk/operational implications,
caveats, reproduction steps. |
| Appendix A/B | Raw DHAT stack frame table + glossary. |
| **§15** | **Verified v2 optimization results** — code changes applied,
type-size impact, DHAT before/after, re-projected scaling. |
| **§16** | **Real-account validation (v2)** — N = 115 partitions on a
real Cosmos INT account, cross-checked against §15 projections. |
| **§17** | **Real-account v1 vs v2 comparison** — apples-to-apples Δ on
the same INT account. |
| **§18** | **Simulated v1 vs v2 at 1k and 10k partitions** —
closed-form model derived from §15/§17 per-entry slopes; predictions for
fleet-scale N. |

### Headline numbers documented

**v1 (HashSet-based) at 80k partitions, fully tripped, 2 failed
regions/partition:**

| Dimension | PPCB Disabled | PPCB Enabled | Delta |
|---|---:|---:|---:|
| Peak live heap | 1,380 B | 24,604,302 B (≈ 23.46 MiB) | +24,602,922 B
|
| Peak live blocks | 13 | 160,014 | +160,001 |
| Bytes / partition entry | — | ~308 B | — |
| Heap blocks / partition entry | — | 2 | — |
| Leaks | 0 | 0 | — |

**v2 (`SmallVec` + `CompactString`) impact:**

- **Block-count pressure at 80k partitions: 160,014 → ~14 (−99.99 %)** —
the dominant operational win (eliminates ~160k allocator round-trips on
partition-event storms).
- **Peak heap at 80k partitions: ~23.5 MiB → ~17 MiB (−28 %)** — also
drops the single contiguous 20 MiB `HashMap` backing allocation,
reducing fragmentation risk.
- **`PartitionFailoverEntry` shrank 8 B** (unexpected secondary win from
`SmallVec`'s `union` feature reusing the discriminant niche).
- Confirmed on a real Cosmos INT account at N = 115 (§16/§17) and
modeled at N = 1k / 10k (§18).

### Reviewer guidance

- **Pure doc PR** — no diff against `Cargo.toml`, `src/`, `tests/`, or
CI.
- The file is 1,571 lines but is fully self-contained; reviewers don't
need to run anything.
- If you want to reproduce, §14 (synthesized harness), §16.10 / §17.7
(real account), and §18.8 (simulated 1k/10k) each have copy-paste
reproduction blocks.
- The code changes the document refers to (v2: `SmallVec` +
`CompactString`) are **already shipped** on
`release/azure_data_cosmos-previews` (see PR #4393 *Slim
PartitionKeyRange cache* in the immediate history) — this PR only adds
the report that motivated and validated them.

### Validation

- `git status` clean after commit (only pre-existing untracked
`sdk/cosmos/.vscode/` remains, untouched).
- Branch is exactly 1 commit ahead of
`origin/release/azure_data_cosmos-previews` (`962c1f7b7`).
- No build/test/clippy/fmt checks needed (Markdown-only change in a
`docs/` folder).

### Out of scope

- Any further memory work beyond `SmallVec` + `CompactString` (e.g.,
§11.3 *boxing the HashMap value* is explicitly deferred).
- Driver source changes (the v2 optimizations are already merged
separately).

Author: kundadebdatta