Enhance "LTS every year" section

Added definitions for Alpha and Beta phases, including stability expectations and release frequency details.

release-schedule-proposal/README.md

@@ -28,18 +28,37 @@ Here are the release schedule proposals:
 
 ## LTS every year
 
-  1. alpha duration: 5 months
-  2. beta duration: 1 months
-  3. current duration: 6 months
-  4. active duration: 12 months
-  5. maintenance duration: 18 months
+  1. Alpha duration: 5 months
+  2. Beta duration: 1 months
+  3. Current duration: 6 months
+  4. Active duration: 12 months
+  5. Maintenance duration: 18 months
 
   Total life span (from start of beta until end of maintenance): 37 months
 
   ![Release schedule proposal preview](./lts-every-year.svg)
 
   ![Release schedule proposal preview](./lts-every-year2030.svg)
 
+### Alpha and Beta definition
+
+#### Stability expectations
+
+Alpha releases may contain semver-major changes (assuming such change does not "break the ecosystem"), but no ABI-breaking changes (so native addons can be tested on those versions).
+
+Once the release has reached the beta period, semver-major changes will no
+longer be included, except:
+
+- security fixes
+- V8 update
+- one-time exception granted by the TSC
+
+Updating V8 during the Beta period is considered the optimal tradeoff, despite
+the ABI and API potential breakage, as otherwise the included V8 version would
+be 1 year old by the time the release line transitions to LTS.
+
+#### Release frequency
+
 Version | Tentative Date | Type of changes
 --------|--------------| -------------
 v27.0.0-alpha.0 |  2026-10-20  | Branch-off
@@ -53,24 +72,27 @@ v27.0.0-alpha.7 |  2027-01-26  | most changes[^1]
 v27.0.0-alpha.8 |  2027-02-09  | most changes[^1]
 v27.0.0-alpha.9 |  2027-02-23  | most changes[^1]
 v27.0.0-alpha.10 |  2027-03-09  | most changes[^1]
-v27.0.0-beta.0 |  2027-03-23  | most changes[^1] + V8 update[^2]
-v27.0.0-beta.1 |  2027-04-06  | semver-minor only + V8 update[^2]
+v27.0.0-beta.0 |  2027-03-23  | most changes[^1] + V8 update
+v27.0.0-beta.1 |  2027-04-06  | semver-minor only + V8 update
 v27.0.0 |  2027-04-20  | semver-minor only
 v27.x.x |  2027-05-04  | …
 v27.x.x |  2027-10-05  | …
 v27.x.x (LTS transition) |  2027-10-20  | 
-(Maintenance transition) |  2028-10-20  | _No release_
-(End-of-Life) |  2030-10-20  | _No release_
+(Maintenance transition) |  2028-10-20  | 
+(End-of-Life) |  2030-10-20  | 
+
+[^1]: Most changes from `main`, except those marked as `dont-land-on-v27.x` because e.g. they contain ABI-breaking changes, or are considered too breaking for the ecosystem, or are moving a deprecation to EOL if 27.x already contains the change moving it to Runtime.
+
+#### Security support
 
-**Security support**: It has not been decided whether security fixes would
+TL;DR: TBD
+
+It has not been decided whether security fixes would
 trigger a security release, or if those would simply be included in the next
 regular release (potentially ahead of schedule). The difference on a maintenance
 perspective is that a security release requires locking the CI until all the
 queued security releases are ready.
 
-[^1]: Most changes from `main`, except those marked as `dont-land-on-v27.x` because they contain ABI-breaking changes or e.g. are moving a deprecation to EOL if 27.x already contains the change moving it to Runtime.
-[^2]: V8 update are (typically) ABI-breaking, however it is considered worth doing given otherwise the V8 version included would be 1 year old by the time the release line transitions to LTS.
-
 ## Longer life cycle
 
   1. current duration: