sumdb/tlog: fix TileHashReader authentication bypass

TileHashReader's ReadHashes operates in five stages:

1. it collects the tiles needed to compute the tree hash (stx);
2. it collects the tiles needed to compute the requested indexes;
3. if fetches all the tiles from the TileReader;
4. it authenticates the stx tiles against the tree hash;
5. it authenticates the remaining tiles against their parents.

At the collection stage, it deduplicates. So for example if h = 2,
N = 15, and indexes = [(0, 01)]

                s3
          ┌───────┴───────┐
          ∘               ∘              s2        <- 1/000.p/3
      ┌───┴───┐       ┌───┴───┐       ┌───┴───┐
      ∘       ∘       ∘       ∘       ∘       ∘      s1     s0
    ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐    |
    00  01  02  03  04  05  06  07  08  09  10  11  12  13  14

    └── 0/000 ───┘  └── 0/001 ───┘  └── 0/002 ───┘ └ 0/003.p/3 ┘

then there are four hashes needed to compute the tree hash
(s3, s2, s1, s0) which sit in two tiles (1/000.p/3 and 0/003.p/3).

Step 4 authenticates those two tiles against the tree hash. Then step 5
should have authenticated tiles[2:] (the remaining tiles) against their
parents. Instead, it was authenticating tiles[len(stx):], so tiles[4:].

That means that tiles[2:4] were never authenticated, but were still
(correctly) used to find hashes for indexes. Those hashes were returned
completely attacker-controlled. The tiles were also passed
attacker-controlled to SaveTiles, breaking its security contract.

Ooof.

Add a comprehensive test that iterates over all tree sizes in [1, 100]
and tries to tamper with every index.

Fixes CVE-2026-56865
Fixes #80744

Change-Id: I4948dfe06e5c8ba068f7f7edfc215c086a6a6964
Reviewed-on: https://go-review.googlesource.com/c/mod/+/814960
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>