[mgravell]

Implements the HybridCache-side spec for per-command local-cache serve/store control, plus the external-cache-layer scenario discussed alongside it — with one deliberate split, below.

Two new public CommandFlags (bits 23–24, per the spec)

• CacheNoLocalServe (1 << 23) — always a round trip. No probe, and no attaching to an identical request already in flight (that reply answers a question asked before this one). Everything else stands: the reply is still eligible for admission, and a RemovalHandler still arms on the fill — including when the store loses to a resident still-valid entry, which is first-fill-wins as for any racing fill.
• CacheNoLocalStore (1 << 24) — an entry already present may be served (serving is not storing; the serve re-arms as usual), but the reply begins no fill at all: no store, no removal enrollment, and no single-flight leadership — a waiter attached to a reply that will never be stored would wake, miss, and send anyway, having gained only the wait.
• Together: full non-participation for that call. Both are inert on a non-participating command, CacheOptOut included (the reserved-pair rejection stays OptIn|OptOut only). Neither has a wire half in any tracking mode. Both are in UserSelectableFlags, and the bits adjoin in HC's read-then-write order, so the HC side's branch-free (CommandFlags)(((int)flags & 0b11) << 23) mapping holds.

The external layer is a policy, not the flag pair

The spec pins CacheNoLocalStore as "nothing stored ⇒ no enrollment arises", which is right for HC-on-CSC — and exactly backwards for the external-layer scenario (FusionCache, the IDistributedCache-based HybridCache), which is enrollment without storage. Overloading one flag with both meanings would poison each, so the read-coupled external shape ("Shape A") lives where the coupling already does:

var external = db.Context.WithCachePolicy(new CachePolicy
{
    Storage = CacheStorage.External,
    RemovalHandler = static (key, reason) => EvictFromOuterCache(key),
});

• The fill runs and refuses only the store: keys registered and tracked, tracked stays true, the handler arms — the same path that already arms on a raced or too-large fill, because the server is tracking either way.
• The read implies CacheOptIn (unless the call said CacheOptOut): a read whose purpose is enrollment must not be silently dropped by the admission doorkeeper or an opt-in mode's default-off. Under TrackingOptIn/TrackingByPrefix it therefore goes out behind CLIENT CACHING YES as it must.
• No in-flight registration is made for such a fill (new TryBeginFill overload takes the policy).
• Entries other callers stored may still be served to it — current per the same invalidation stream — and the serve re-arms. LocalWrite arrives distinctly: the one removal whose replacement value the outer layer already has.
• On CacheOptions.DefaultPolicy it is connection-wide. Per-call flags still win: CacheNoLocalStore skips the fill (and the enrollment) for that one command.
• New counter: RefusedExternal.

The broadcast-shaped integration ("Shape B": a raw connection-level invalidation stream over CLIENT TRACKING BCAST, no read coupling) is not in this PR — it is a CacheOptions-level surface of its own, and can land independently.

The ToString() trap, as foretold

Declaring the new members beside the bit 20–22 cluster flipped which of the duplicate-valued names (DemandSlave/DemandReplica) ToString() picks — precisely the perturbation the enum's warning comment describes, caught by FormatTests.CommandFlagsFormatting. Fixed by declaration position: a brute-force check of every insertion point (scratch program over all 22 candidate layouts) shows positions 5–9 and end-of-enum keep the expected formatting on net8.0 and net10.0; they are declared at the end, with a pointer comment left in the bit cluster. netfx formatting is asserted by the same test on Windows CI.

Verification

• 12 new tests (RespCacheParticipationTests): warm/cold serve- and store-disabled behaviour, both-flags non-participation, inertness alongside CacheOptOut, external mode enrolling from the first read under default (doorkeeper) admission, re-arm on re-read, serving from others' entries, LocalWrite, connection-wide default policy, TrackingOptIn mode, and the flag-beats-policy case.
• Full cache/tracking/stale/format sweep: 430/430 on net10.0; formatting also green on net8.0. Library builds clean on all six TFMs (PublicAPI updated).
• docs/ClientSideCaching.md: the two flags in the per-call section and the mode table's follow-up paragraph, a Storage row in the policy table, and a new "Feeding an external cache layer" section with the re-fetch-not-deleted caveat (the server sheds its bounded tracking table by invalidating).