docs(hydro_lang): restructure reference docs around sliced blocks and mutable singleton references

[shadaj]

Reorganize the "Core Framework" reference docs into a new hierarchy and
rewrite content to reflect current Hydro APIs (sliced! blocks,
by_ref/by_mut reference handles, BoundedValue keyed singletons)
instead of the legacy .tick()-era APIs.

New structure under docs/docs/hydro/reference/:

• introduction/: Introduction (keeps /docs/hydro/reference/ URL via
  slug), new "Live Collections" concept page (placed first, as the most
  core concept), and "Dataflow Programming" (refreshed to emphasize that
  Hydro looks like analytics frameworks but models request/response
  services)
• correctness/ ("Safety and Correctness"): section index; "Bounded and
  Unbounded Types" rewritten to decouple boundedness from ticks (Bounded =
  complete, contents immediately available; unbounded→bounded conversion
  via sliced! hooks); "Eventual Determinism" trimmed to the semantic
  model; new flagship "Non-Determinism and nondet!" page covering guard
  mechanics, locally-resolved vs forwarded guards, multiple NonDet
  parameters as independently observable forms of non-determinism (Paxos
  example), proof-style comment guidance, nondet_ naming and
  # Non-Determinism rustdoc conventions, simulator tie-in, and
  user-defined-function caveats
• streaming-data/ ("Streaming Data"): section index (request/response
  framing); "Streams" (refreshed); "Keyed Streams" (hidden doctest
  harnesses migrated to sliced!); new "Keyed Singletons" page focused on
  the BoundedValue variant (one immutable value per request key;
  first(), fold_early_stop, map-based request→response, slicing as a
  batch of new entries) with a new request/response animation
• state-management/ ("State Management"): new section index laying out
  the state philosophy (declarative aggregations → sliced+by_ref reads →
  use::state+by_mut for multi-input imperative state); "Singletons and
  Optionals" written from placeholder; new "Keyed State" page (Unbounded
  KeyedSingleton, per-key aggregations, join/lookup patterns); "Slice
  Blocks" refreshed (leads with the by_ref snapshot-reading idiom); new
  "Slice Hooks" reference (use / use::atomic / use::state /
  use::state_null semantics per collection type, guarantees, unslicing);
  new "References and Mutations" page (by_ref/by_mut handles,
  boundedness requirement, code-order access groups, multi-input
  shared-state pattern)
• atomic-collections.mdx: "Atomic Collections" promoted to a top-level
  doc, de-tick-ified wording, modern sim test API in examples

Also update all internal links (learn/, dfir/, locations/),
regenerate category files and sidebar positions, and update hydro.run doc
URLs in hydro_lang rustdoc comments to the new paths.

Validation: full docusaurus build passes (no broken links); all mdtest
doctests pass (62 passing, remainder intentionally no_run/ignore); no
legacy APIs (.tick()/.batch(&tick)/.snapshot(&tick)/.all_ticks()/
.latest()) appear in any visible example code.

Co-authored-by: Infinity 🤖 infinity@hydro.run

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View logs

[luckyworkama]

under streaming data we have "streams, keyed streams, keyed singletons", then under the next section, state management, it starts with "singletons".

Something feels wrong about that order

[shadaj]

under streaming data we have "streams, keyed streams, keyed singletons", then under the next section, state management, it starts with "singletons".

Something feels wrong about that order

So the tricky part is keyed singletons can be used in both "stream mode" (ValueBounded) and "state mode" (Unbounded, MonotoneValue, etc). So we kinda have to talk about them twice in the two different places. But open to better titles.

[shadaj]

under streaming data we have "streams, keyed streams, keyed singletons", then under the next section, state management, it starts with "singletons".
Something feels wrong about that order

So the tricky part is keyed singletons can be used in both "stream mode" (ValueBounded) and "state mode" (Unbounded, MonotoneValue, etc). So we kinda have to talk about them twice in the two different places. But open to better titles.

I renamed that page to Streaming Keyed Singletons.