fix: Read category index instead of tag index in category_stats

[Samuel1505]

Description

// analytics.rs:50-59
pub fn category_stats(env: &Env, category_id: u32) -> CategoryStats {
    ...
    // Iterate through tag index to find grants in this category
    let grant_ids = Storage::get_tag_index(env, category_id);
    ...
}

Storage::get_tag_index (storage/helpers.rs, line 927) reads DataKey::Grant(GrantKey::TagIndex(u32)), which is populated only by grant_tags::hash_tag's hash of a freeform tag string — a 32-bit hash value, not a small sequential category id. The correct per-category index is Storage::get_category_index (storage/helpers.rs, line 953), keyed by GrantKey::CategoryIndex(u32), which is exactly what grant_tags::find_by_category correctly uses elsewhere in the codebase.

Since a small integer category_id (0, 1, 2, …) will essentially never collide with a 32-bit tag hash, get_tag_index(category_id) returns an empty list for every real category. Any number of grants can be correctly tagged into a category via grant_tags::tag_grant, but analytics_category_stats(category_id) will always report total_grants = 0, completed_grants = 0, total_funded = 0, success_rate_bps = 0. This also silently breaks build_snapshot's "top category by funding" logic (which calls category_stats internally and picks the category with the highest total_funded, initialized to 0) — top_category_id can never be selected, with no error surfaced to callers.

Technical Requirements

Files to update

• contracts/contracts/stellar-grants/src/analytics.rs (category_stats, lines 50-93)

Fix direction

let grant_ids = Storage::get_category_index(env, category_id);

Acceptance Criteria

• analytics_category_stats(category_id) correctly reflects grants tagged into that category via grant_tags::tag_grant.
• A test tags several grants into a category, calls category_stats, and asserts total_grants/total_funded match expectations (nonzero).
• build_snapshot's top-category selection works correctly once fed real per-category data.
• cargo test passes.

Estimated Effort

Beginner: 2 hours
Intermediate: 1 hour
Expert: 0.5 hours

How to work this issue

1. Read contracts/ContributionGuide.md for the contribution workflow.
2. Comment on the issue to claim it before starting.
3. Branch: fix/issue-931-analytics-category-index.
4. Run cargo fmt, cargo clippy -- -D warnings, cargo test before opening your PR.
5. Use a Conventional Commit message, e.g. fix: read category index instead of tag index in category_stats.

Before you start

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